Saving all attributes of LabelModel

[paroma]

Description of proposed changes

Saving and loading LabelModel via attribute __dict__ pickling. This saves out parameters other than weights like mu that are needed to methods like predict() without having to call fit() with the loaded model

Related issue(s)

Fixes #1460

Test plan

Edited test for save() and load() to check predictions are the same for saved and loaded model

Checklist

• I have read the CONTRIBUTING document.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have run tox -e complex and/or tox -e spark if appropriate.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #1463 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1463      +/-   ##
==========================================
+ Coverage   97.58%   97.58%   +<.01%     
==========================================
  Files          55       55              
  Lines        2029     2032       +3     
  Branches      334      334              
==========================================
+ Hits         1980     1983       +3     
  Misses         22       22              
  Partials       27       27

Impacted Files	Coverage Δ
snorkel/labeling/model/label_model.py	95.8% <100%> (+0.04%)	⬆️

[bhancock8]

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