improve Cosmology init signatures - #11906

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

Blocking #11559
Suggested labels: no-changelog-needed, merge-when-CI-passes
Request Review: @mhvk (from discussion in #11559 )

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.
• Is a milestone set? Milestone must be set but astropy-bot check might be missing; do not let the green checkmark fool you.
• If the milestone set requires a backport to release branch(es), apply the appropriate backport-X.Y.x label(s) before merge.