Update distinct to be more like mongo distinct

[shyamd]

This PR updates distinct to be more like mongo's distinct. It primarily removes the multi-field distinct.

[codecov]

Codecov Report

Merging #92 into master will decrease coverage by 0.1%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   87.07%   86.97%   -0.11%     
==========================================
  Files          20       20              
  Lines        1447     1443       -4     
==========================================
- Hits         1260     1255       -5     
- Misses        187      188       +1

Impacted Files	Coverage Δ
src/maggma/stores/aws.py	76.41% <0%> (ø)	⬆️
src/maggma/stores/mongolike.py	93.71% <100%> (-0.48%)	⬇️
src/maggma/stores/gridfs.py	84.37% <100%> (-0.13%)	⬇️
src/maggma/stores/compound_stores.py	80.79% <100%> (+0.34%)	⬆️
src/maggma/core/store.py	87.23% <100%> (-0.53%)	⬇️
src/maggma/stores/advanced_stores.py	80.55% <66.66%> (-0.56%)	⬇️
src/maggma/builders/group_builder.py	85% <0%> (ø)	⬆️

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[lgtm-com]

This pull request introduces 1 alert when merging 60de246 into 5d1b7fc - view on LGTM.com

new alerts:

• 1 for First argument to super() is not enclosing class