FileStore: fix metadata overwriting path

[rkingsbury]

When a user adds metadata to FileStore it is stored on disk. Both the file_id and the path are retained in the .json file to make it possible to manually associated metadata with specific files. However, when the metadata was read into the FileStore, the path attribute stored on disk was overwriting the path attribute determine by FileStore.__init__. This was not intended behavior, and caused problems when working with a FileStore across different computers where the absolute path of files is different (but the path relative to the FileStore root is the same). (For example, syncing a directory of files containing a FileStore across different computers using Dropbox, etc.)

This PR fixes the bug as follows:

• Store a new field path_relative instead of path in the metadata .json file on disk. This preserves manual identifiability of metdata
• Ensure that the path and last_updated fields cannot be overwritten by metadata updates
• add tests

[codecov]

Codecov Report

Base: 88.37% // Head: 88.37% // No change to project coverage 👍

Coverage data is based on head (5032524) compared to base (fe2a735).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #736   +/-   ##
=======================================
  Coverage   88.37%   88.37%           
=======================================
  Files          41       41           
  Lines        3123     3123           
=======================================
  Hits         2760     2760           
  Misses        363      363           

Impacted Files	Coverage Δ
src/maggma/stores/file_store.py	98.02% <100.00%> (ø)

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[munrojm]

This looks good! Ready to merge?

[rkingsbury]

This looks good! Ready to merge?

Yes, ready!