To continue in the effort of merging Sam Shaner's transient capability,
this next PR adds the class DiffusionCoefficient to mgxs.py. It uses
the TransportXS class, but has some unique features in get_condensed_xs()
requiring its own function. There won't be any other MGXS classes added
after this.
I updated the docstrings and the appropriate regression tests.
Since my system outputs different formatting for the
mgxs_library_distribcell test, it is possible this test will continue
to fail. However, I did my best to modify it by hand so that it will pass.
This test often fails due to the formating of the results.
Now, I checked manually, and all these results are identical,
except of course for removing the "group" variable on the penultimate
chunk of results which is decay-rate.
So hopefully this formatting I get on my machine passes the other systems as well.
