cosmo: improve mapping io and tests - #12209

Cosmology instance conversion to dict now accepts keyword argument cls to
determine dict type, e.g. OrderedDict. This argument is largely driven by to/from_table (#12213) wherein __astropy_table__ allows for a cls argument. Implementing that in to_table meant doing so here as well. And I think it's the better for it.

Also the tests are much improved.
I think the partial information test is impossible to generalize, but this new format of test class inheritance means each format can provide a partial-information test that will be used to test the to/from_X function as well as Cosmology.to/from_format() for each Cosmology subclass.

Signed-off-by: Nathaniel Starkman (@nstarman) [email protected]

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the Astropy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see "When to rebase and squash commits".
• Did the CI pass? If no, are the failures related? If you need to run daily and weekly cron jobs as part of the PR, please apply the Extra CI label.
• Is a change log needed? If yes, did the change log check pass? If no, add the no-changelog-entry-needed label. If this is a manual backport, use the skip-changelog-checks label unless special changelog handling is necessary.
• Is a milestone set? Milestone must be set but astropy-bot check might be missing; do not let the green checkmark fool you.
• At the time of adding the milestone, if the milestone set requires a backport to release branch(es), apply the appropriate backport-X.Y.x label(s) before merge.