Add pincell depletion jupyter notebook

The basic structure is borrowed from examples/python/pincell_depletion/
and builds a single pin cell to be depleted. The model building is
expedited, as this assumes the user has read through the
basic user guide on model building. The new openmc.model.pin
function is highlighted, both for building a plain pin
and for subdividing regions for depletion.

The Chain and Operator classes are the focuses of the example,
and the PredictorIntegrator is used to deplete the pin out to
6 months using 30 day depletion steps with 174 W power.
The openmc-make-depletion-chain scripts are introduced as methods
for building initial depletion chains. Use of
openmc.data.DataLibrary.register_file is included, as this allows
the user to not pass the chain_file argument if the depletion chain
is included in OPENMC_CROSS_SECTIONS.

The jupyter notebook is included in the documentation similar
to other examples.

docs/source/examples/index.rst

@@ -24,6 +24,7 @@ General Usage
     nuclear-data
     nuclear-data-resonance-covariance
     cad-geom
+    pincell-depletion
 
 --------
 Geometry

docs/source/examples/pincell-depletion.rst

@@ -0,0 +1,14 @@
+.. _notebook_depletion:
+
+=================
+Pincell Depletion
+=================
+
+
+.. only:: html
+
+    .. notebook:: ../../../examples/jupyter/pincell_depletion.ipynb
+
+.. only:: latex
+
+    IPython notebooks must be viewed in the online HTML documentation.