[BREAKING] Write out results to JSON instead of pickle

[Andrew-S-Rosen]

Summary of Changes

Write out the results to a JSON file instead of a pickle file to prevent potential security issues. It can be read in as:

from monty.serialization import loadfn

results = loadfn("quacc_results.json.gz")

It can also be viewed in any text editor.

Checklist

• I have read the "Guidelines" section of the contributing guide. Don't lie! 😉
• My PR is on a custom branch and is not named main.
• I have added relevant, comprehensive unit tests.

Notes

• Your PR will likely not be merged without proper and thorough tests.
• If you are an external contributor, you will see a comment from @buildbot-princeton. This is solely for the maintainers.
• When your code is ready for review, ping one of the active maintainers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (56c1caf) to head (88cd57a).

❗ Current head 88cd57a differs from pull request most recent head 6dc814c. Consider uploading reports for the commit 6dc814c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2025   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          81       81           
  Lines        3339     3339           
=======================================
  Hits         3307     3307           
  Misses         32       32           

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