Import galacticus subhalos example

[cgannonucm]

Implements the following changes

• Added example_notebooks/from_galacticus.ipynb
• Added an id property to track galacticus subhalos

[dangilman]

@cgannonucm could you please add test functions for the classes and methods you created? You can have a look at the test suites in pyHalo located in pyhalo/tests

[dangilman]

@cgannonucm I think the tests are failing because the syntax you used to merge dictionaries isn't supported in python 3.8

[dangilman]

@cgannonucm ok everything looks great! Do you need to make any changes to the example notebook so that it is compatible with the latest changes? If not, then I'll go ahead and merge this

[cgannonucm]

The notebooks should be good to go! I would like to push one more commit, before we merge. I am going to add the parameter file used to generate the (galacticus) output file.

[cgannonucm]

@dangilman Everything should be good to go after tests finish up.