Updated GroupBuilder

[shyamd]

This PR updates Group Builder to be a bit easier to use and add in documentation

• Simpler group builder implementation
• Some basic tests
• Docs

Some misc fixes:

• include tqdm progress bar in Builder.run
• Actually re-run failed keys

[codecov]

Codecov Report

Merging #79 into master will increase coverage by 0.6%.
The diff coverage is 86.27%.

@@            Coverage Diff            @@
##           master      #79     +/-   ##
=========================================
+ Coverage   86.13%   86.73%   +0.6%     
=========================================
  Files          18       20      +2     
  Lines        1298     1365     +67     
=========================================
+ Hits         1118     1184     +66     
- Misses        180      181      +1

Impacted Files	Coverage Δ
src/maggma/core/builder.py	92.59% <100%> (+0.92%)	⬆️
src/maggma/stores/__init__.py	100% <100%> (ø)	⬆️
src/maggma/cli/multiprocessing.py	93.93% <100%> (+0.09%)	⬆️
src/maggma/builders/__init__.py	100% <100%> (ø)
src/maggma/builders/map_builder.py	91.2% <77.77%> (ø)
src/maggma/builders/group_builder.py	84.81% <84.81%> (ø)

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[lgtm-com]

This pull request introduces 1 alert when merging 3566059 into c263b94 - view on LGTM.com

new alerts:

• 1 for Redundant assignment