Attributes listed in doc strings but missing from properties

After getting an error "XSdata has no attribute 'beta'", I saw that
beta, decay_rate, and inverse_velocity were listed as attributes in
XSdata, but were not included in the @properties.

Just added them. Also noticed there doesn't seem to be a test for this class.

openmc/mgxs_library.py

@@ -349,6 +349,18 @@ class XSdata:
     def chi_delayed(self):
         return self._chi_delayed
 
+    @property
+    def beta(self):
+        return self._beta
+
+    @property
+    def decay_rate(self):
+        return self._decay_rate
+
+    @property
+    def inverse_velocity(self):
+        return self._inverse_velocity
+
     @property
     def num_orders(self):
         if self._order is None: