Add support for reading chandra_models data

[taldcroft]

Description

This is in support of sot/chandra_models#104 and the general effort to move more operations data or model files into chandra_models.

It is based heavily off of similar code in xija.get_xija_model.

While building the sphinx docs I saw a bunch of errors in ska3-prime and so I just fixed them here.

Interface impacts

Adds a new module.

Testing

Unit tests

• Mac (New unit tests)
• TO DO: Run unit testing run on Windows.

Independent check of unit tests by Jean

• Linux

Functional tests

None

[jeanconn]

I think the most useful behavior for me would be if the git mechanism in interacting with the chandra_models repo would confirm that the repo was checked out at or past the most recent tag on github, and then just gave a warning if the repo is dirty or different.

[jeanconn]

If I have a work around of just tagging in chandra_models to do chandra_models testing (noted in sot/ska_sun#27 (comment)) these changes in ska_helpers that basically do what was done in xija make sense to me.