Minor bug fixes to S3Store

[jmmshn]

Fix for unpacking issue

Reconstructing data from msgpack is not compatible with how monty wants to reconstruct objects.
msgpack wants to go as deep as possible first, but monty wants to stop when it finds a from_dict

So now we use MontyDecoder().process_decoded and issue should be resolved.

Doing this also allows us to bypass string conversion (json.dumps) which gives us up to 5x speedup when processing CHGCARs and 1.5x on bandstructures.

Fixed mismatched between documentation and functionality of update function

Now allows for a single document to be updated

[codecov]

Codecov Report

Merging #253 into master will decrease coverage by 0.69%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   88.33%   87.64%   -0.70%     
==========================================
  Files          27       27              
  Lines        2006     2007       +1     
==========================================
- Hits         1772     1759      -13     
- Misses        234      248      +14     

Impacted Files	Coverage Δ
src/maggma/stores/aws.py	80.39% <66.66%> (-0.53%)	⬇️
src/maggma/stores/mongolike.py	87.25% <0.00%> (-6.38%)	⬇️

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[lgtm-com]

This pull request introduces 1 alert when merging 9c78c5a into 3b4832c - view on LGTM.com

new alerts:

• 1 for Unused import