Create shared sfeos helpers module

[jonhealy1]

Related Issue(s):

• Create shared helper module to make elasticsearch/ opensearch code easier to maintain #369

Description:

• This pr moves shared elasticsearch/ opensearch logic out of core and into the sfeos_helpers module.
• Other code duplicated in es/ os database_logic files has been moved into the sfeos_helpers module as well.
• The readme has been updated/ reorganized and a table of contents has been added.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jonhealy1]

This will be a difficult pr to review maybe?

1. The core module should be database-agnostic, so code that relied on es/ os was moved to the best of my ability. I moved the aggregations code to sfeos_helpers but maybe some should have stayed?
2. Code that was shared between both backends was also moved to sfeos_helpers. More code could potentially be moved?
3. The readme was refactored and I hope that all of the information was retained correctly and that the bullet point format improves readability.

[jonhealy1]

50 files changed so yes, difficult to review :)

[jamesfisher-geo]

This is a great change. Maintaining the duplicate code was getting cumbersome. I'll dig into this PR over the weekend 👍

[jonhealy1]

Still a lot of duplicate code, but it's a start hopefully that will help improve maintenance and also keep the core module database agnostic.

[jonhealy1]

Sharing too much code could be troublesome in the future as well as the two projects diverge more from each other.