Rewrite Open Logic

[AstroPatty]

This PR largely rewrites the routines for opening files, creating a much more streamlined system. Advantages include:

• Much easier to add new data and collection types
• Decisions about what to load are made in a single place
• Singifincant improvements to open speed, particularly when working with large numbers of files

The only user-facing change is that opencosmo.open now accepts multiple files. As a result, opencosmo.open_linked_files has been marked as depcrecated.

Additionally, this PR adds an "overwrite" flag to opencosmo.write, which allows users to overwrite files that already exist. Thanks @gideonkmc for the suggestion