update gridfs store

[gpetretto]

Currently the GridFSStore does not allow to access data from gridfs collections that were not generated with maggma, since they will not contain the metadata value. This PR aims to allow the retrieval of data from a generic gridfs collection.

In its current form however the method returns the bytes extracted from the gridfs and not a dict, as required by the API. In general this could happen if this json.loads fails:
https://github.com/materialsproject/maggma/blob/master/src/maggma/stores/gridfs.py#L209

                try:
                    data = json.loads(data)
                except Exception:
                    pass

Would you agree to also replace this with:

                try:
                    data = json.loads(data)
                except Exception:
                    if not isinstance(data, dict):
                        data = dict(data=data)

?

[codecov]

Codecov Report

Merging #381 (cde6c91) into master (e0c319b) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
- Coverage   86.85%   86.74%   -0.12%     
==========================================
  Files          28       28              
  Lines        2222     2225       +3     
==========================================
  Hits         1930     1930              
- Misses        292      295       +3     

Impacted Files	Coverage Δ
src/maggma/stores/gridfs.py	90.90% <100.00%> (+1.54%)	⬆️
src/maggma/stores/mongolike.py	82.12% <0.00%> (-1.91%)	⬇️
src/maggma/cli/serial.py	79.31% <0.00%> (ø)
src/maggma/cli/source_loader.py	70.11% <0.00%> (ø)
src/maggma/cli/multiprocessing.py	94.64% <0.00%> (ø)
src/maggma/cli/__init__.py	92.00% <0.00%> (+0.16%)	⬆️

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[shyamd]

You might also include the key field and the last_updated data from the index. Please update the docstrings to make it clear that is what gets returned.

[gpetretto]

@shyamd I hope I interpreted your suggestion correctly.
I also added a test for this specific use case.