added script for openmc lib example that changes material on a parameter study

[shimwell]

This script performs many simulations to find Tritium Breeding Ratio (TBR)
as a function of lithium 6 enrichment while only loading the nuclear data for
the nuclides just once.

As the simulation for TBR is quick and the loading of nuclear data is a relatively
large part of the process this saves a significant amount of time.
Using just the standard openmc python API one would have to load the same
nuclear data for each simulation.

openmc.lib provides Python bindings to the C/C++ API so we can have fine grain
control of the loading of the data, accessing the tallies and changing materials.