Added IPython notebook with post-processing examples.

Since the notebook shows an example of how to histogram relative errors from a
statepoint, we don't really need the openmc-statepoint-histogram utility, so
it's been deleted. The user's guide section on post processing has been updated
as well.

docs/source/pythonapi/examples/post-processing.rst

@@ -0,0 +1,13 @@
+.. _notebook_post_processing:
+
+===============
+Post Processing
+===============
+
+.. only:: html
+
+    .. notebook:: post-processing.ipynb
+
+.. only:: latex
+
+    IPython notebooks must be viewed in the online HTML documentation.

docs/source/pythonapi/index.rst

@@ -62,6 +62,7 @@ on a given module or class.
 .. toctree::
     :maxdepth: 1
 
+    examples/post-processing
     examples/pandas-dataframes
     examples/tally-arithmetic