Add generic indexer

[jhp-lanl]

PR Summary

This MR extracts the GenericIndexer that I created in #526 into its own MR.

I would like to use this capability for the EAP integration effort so that I have a bit more flexibility with allocating lambda arrays when compared to the CacheAccessor that already exists with the closure models. That class hard codes the number of lambdas, whereas I'd like that to be something I can set.

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Move the changes in the CHANGELOG.md file under a new header for the new release, and reset the categories.
• Ensure that any when='@main' dependencies are updated to the release version in the package.py

[jhp-lanl]

@jonahm-LANL I'm going to leave it to you guys if you want to document this or replace other indexers in the code with this one

[Yurlungur]

Looks good. I think lets get this in now and we can re-use it elsewhere later if we want to.

[jhp-lanl]

I'll run the gitlab tests just to be sure and then merge this