Automatically infer multiclass based off of objective

[jeremyliweishih]

This changes the default behavior of multiclass.

If there is a given objective it will do the following:

• Check if the given objective is compatible with multiclass
• Set problem_type to MULTICLASS if the given objective is EXCLUSIVELY multiclass
• Set problem_type to MULTICLASS if compatible

The default objective will be binary precision and if multiclass is set to TRUE it will default to precision_micro.

Also adds a check so users who end up with a mismatch of objective and multiclass will have an easier error to handle.

[codecov]

Codecov Report

Merging #99 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   93.49%   93.72%   +0.23%     
==========================================
  Files          52       52              
  Lines        1306     1355      +49     
==========================================
+ Hits         1221     1270      +49     
  Misses         85       85

Impacted Files	Coverage Δ
evalml/models/auto_base.py	91.95% <100%> (+0.6%)	⬆️
evalml/objectives/fraud_cost.py	100% <100%> (ø)	⬆️
evalml/tests/automl_tests/test_autoclassifier.py	100% <100%> (ø)	⬆️
evalml/problem_types/problem_types.py	100% <100%> (ø)	⬆️
evalml/objectives/lead_scoring.py	100% <100%> (ø)	⬆️
evalml/models/auto_classifier.py	100% <100%> (ø)	⬆️

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[kmax12]

logic seems good. just some comments on making code a bit cleaner

[kmax12]

LGTM