Creating binary and multiclass objective classes

[angela97lin]

As part of #346, we need to split our objectives into binary and multiclass objectives. Closes #373, addresses #294 as well (since we wanted Accuracy to be our default objective)

[codecov]

Codecov Report

Merging #419 into improved_objectives will decrease coverage by 0.41%.
The diff coverage is 98.07%.

@@                   Coverage Diff                   @@
##           improved_objectives     #419      +/-   ##
=======================================================
- Coverage                98.25%   97.83%   -0.42%     
=======================================================
  Files                      104      115      +11     
  Lines                     3265     3420     +155     
=======================================================
+ Hits                      3208     3346     +138     
- Misses                      57       74      +17

Impacted Files	Coverage Δ
evalml/objectives/__init__.py	100% <ø> (ø)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	100% <100%> (ø)	⬆️
evalml/tests/objective_tests/test_lead_scoring.py	100% <100%> (ø)	⬆️
evalml/objectives/standard_metrics.py	99.51% <100%> (-0.06%)	⬇️
evalml/pipelines/classification/catboost_binary.py	100% <100%> (ø)
evalml/objectives/fraud_cost.py	97.05% <100%> (-2.95%)	⬇️
...egression_pipeline_tests/test_linear_regression.py	100% <100%> (ø)
evalml/pipelines/classification/__init__.py	100% <100%> (ø)	⬆️
...ests/classification_pipeline_tests/test_xgboost.py	100% <100%> (ø)
...ine_tests/classification_pipeline_tests/test_rf.py	100% <100%> (ø)
... and 49 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 91d62ec...7713969. Read the comment docs.

[dsherry]

This is very exciting! I left some suggestions and comments, plus I know we have a discussion scheduled for 30min from now on how binary classification objective optimization should work. Once we do that, I'll try to circle back and add some testing suggestions at some point as well.