Finalized check for mesh & made progress on regression test

I had to put domain properties back into the SurfaceMGXS class to perform a checktype on the domain_type.
I updated the regression test mgxs_library_mesh to expect the correct input.
However, now the test fails for a 2-dimensional mesh. In 3D, the Current MGXS fails because it does not contain a 'z' dimension.
Meanwhile, all the other classes contain a 'z' dimension, equal to 1.
I need to figure out why the mesh used by SurfaceMGXS doesn't contain the same information as the other classes, even though it is the same mesh used in all.

openmc/mgxs/mgxs.py

@@ -1964,6 +1964,7 @@ class MGXS:
         # energy groups such that data is from fast to thermal
         if self.domain_type == 'mesh':
             mesh_str = 'mesh {0}'.format(self.domain.id)
+            print("HELLO WORLD", mesh_str)
             df.sort_values(by=[(mesh_str, 'x'), (mesh_str, 'y'),
                                (mesh_str, 'z')] + columns, inplace=True)
         else:
@@ -5979,16 +5980,33 @@ class SurfaceMGXS(MGXS):
                                           by_nuclide, name)
         self._estimator = ['analog']
         self._valid_estimators = ['analog']
-        if domain_type != 'mesh':
-            msg = 'Unable to compute a SurfaceMGXS for domain type {0} ' \
-                    'which is not a mesh type'.format(domain_type)
-            raise ValueError(msg)
-
 
     @property
     def scores(self):
         return [self.rxn_type]
 
+    @property
+    def domain(self):
+        return self._domain
+
+    @property
+    def domain_type(self):
+        return self._domain_type
+
+    @domain.setter
+    def domain(self, domain):
+        cv.check_type('domain', domain, openmc.RegularMesh)
+        self._domain = domain
+
+        # Assign a domain type
+        if self.domain_type is None:
+            self._domain_type = 'mesh'
+
+    @domain_type.setter
+    def domain_type(self, domain_type):
+        cv.check_value('domain type', domain_type, 'mesh')
+        self._domain_type = domain_type
+
     @property
     def filters(self):
         group_edges = self.energy_groups.group_edges
@@ -6036,18 +6054,8 @@ class SurfaceMGXS(MGXS):
                   'linked with a summary file'
             raise ValueError(msg)
 
-        # Use tally "slicing" to ensure that tallies correspond to our domain
-        # NOTE: This is important if tally merging was used
-        if self.domain_type == 'mesh':
-            filters = []
-            filter_bins = []
-        elif self.domain_type != 'distribcell':
-            filters = [_DOMAIN_TO_FILTER[self.domain_type]]
-            filter_bins = [(self.domain.id,)]
-        # Distribcell filters only accept single cell - neglect it when slicing
-        else:
-            filters = []
-            filter_bins = []
+        filters= []
+        filter_bins = []
 
         # Clear any tallies previously loaded from a statepoint
         if self.loaded_sp:
@@ -6126,6 +6134,9 @@ class SurfaceMGXS(MGXS):
                 subdomain_bins.append(subdomain)
             filter_bins.append(tuple(subdomain_bins))
 
+        xs = self.xs_tally.get_values(filters=filters,
+                filter_bins=filter_bins, value=value)
+
         # Construct list of energy group bounds tuples for all requested groups
         if not isinstance(groups, str):
             cv.check_iterable_type('groups', groups, Integral)

tests/regression_tests/mgxs_library_mesh/inputs_true.dat

@@ -53,7 +53,10 @@
   <filter id="28" type="legendre">
     <order>3</order>
   </filter>
-  <filter id="65" type="delayedgroup">
+  <filter id="66" type="meshsurface">
+    <bins>1</bins>
+  </filter>
+  <filter id="69" type="delayedgroup">
     <bins>1 2 3 4 5 6</bins>
   </filter>
   <tally id="1">
@@ -363,61 +366,67 @@
     <estimator>analog</estimator>
   </tally>
   <tally id="52">
+    <filters>66 2</filters>
+    <nuclides>total</nuclides>
+    <scores>current</scores>
+    <estimator>analog</estimator>
+  </tally>
+  <tally id="53">
     <filters>1 2</filters>
     <nuclides>total</nuclides>
     <scores>flux</scores>
     <estimator>tracklength</estimator>
   </tally>
-  <tally id="53">
-    <filters>1 65 2</filters>
+  <tally id="54">
+    <filters>1 69 2</filters>
     <nuclides>total</nuclides>
     <scores>delayed-nu-fission</scores>
     <estimator>tracklength</estimator>
   </tally>
-  <tally id="54">
-    <filters>1 65 2</filters>
-    <nuclides>total</nuclides>
-    <scores>delayed-nu-fission</scores>
-    <estimator>analog</estimator>
-  </tally>
   <tally id="55">
-    <filters>1 65 5</filters>
+    <filters>1 69 2</filters>
     <nuclides>total</nuclides>
     <scores>delayed-nu-fission</scores>
     <estimator>analog</estimator>
   </tally>
   <tally id="56">
+    <filters>1 69 5</filters>
+    <nuclides>total</nuclides>
+    <scores>delayed-nu-fission</scores>
+    <estimator>analog</estimator>
+  </tally>
+  <tally id="57">
     <filters>1 2</filters>
     <nuclides>total</nuclides>
     <scores>nu-fission</scores>
     <estimator>tracklength</estimator>
   </tally>
-  <tally id="57">
-    <filters>1 65 2</filters>
-    <nuclides>total</nuclides>
-    <scores>delayed-nu-fission</scores>
-    <estimator>tracklength</estimator>
-  </tally>
   <tally id="58">
-    <filters>1 65 2</filters>
+    <filters>1 69 2</filters>
     <nuclides>total</nuclides>
     <scores>delayed-nu-fission</scores>
     <estimator>tracklength</estimator>
   </tally>
   <tally id="59">
-    <filters>1 65 2</filters>
+    <filters>1 69 2</filters>
+    <nuclides>total</nuclides>
+    <scores>delayed-nu-fission</scores>
+    <estimator>tracklength</estimator>
+  </tally>
+  <tally id="60">
+    <filters>1 69 2</filters>
     <nuclides>total</nuclides>
     <scores>decay-rate</scores>
     <estimator>tracklength</estimator>
   </tally>
-  <tally id="60">
+  <tally id="61">
     <filters>1 2</filters>
     <nuclides>total</nuclides>
     <scores>flux</scores>
     <estimator>analog</estimator>
   </tally>
-  <tally id="61">
-    <filters>1 65 2 5</filters>
+  <tally id="62">
+    <filters>1 69 2 5</filters>
     <nuclides>total</nuclides>
     <scores>delayed-nu-fission</scores>
     <estimator>analog</estimator>