Add psmc xija

@taldcroft 

This replaces the nmass PSMC model with the Xija version.

I reviewed the outputs of ``test_plot_constraints.py`` and ``pyger sim``  for this branch and the current master branch and observed only expected differences.

I updated the docs and example script to include the newer models. I am not entirely happy with the defaults I chose for the example plots, however given all the changes already made, I would rather tweak the defaults in a separate update.

I apologize for including the autopep8 updates in this pull request. I got carried away with this and forgot you suggested I do this in a separate pull request. I didn't realize it would make the composite view of all the changes so difficult to read; perhaps it would be easier to look at the changes on a commit by commit basis this time around.

Please let me know if you have any questions.