Add Balanced Accuracy Metric

[gsheni]

Pull Request Description

• Closes Add Balanced Accuracy Metric #611

[dsherry]

@gsheni do you have time to finish this off this week?

@angela97lin just merged changes to how we define objectives, adding
a BinaryClassificationObjective subclass. Check out her PR #624 for an example on how to update this PR after those changes. Specifically, you'll now need to subclass BinaryClassificationObjective

[gsheni]

@dsherry
Yah I can get to it this week.

[codecov]

Codecov Report

Merging #612 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #612   +/-   ##
=======================================
  Coverage   98.96%   98.96%           
=======================================
  Files         133      133           
  Lines        4534     4545   +11     
=======================================
+ Hits         4487     4498   +11     
  Misses         47       47           

Impacted Files	Coverage Δ
evalml/objectives/__init__.py	100.00% <ø> (ø)
evalml/objectives/utils.py	94.44% <ø> (ø)
evalml/objectives/standard_metrics.py	99.53% <100.00%> (+0.01%)	⬆️
evalml/tests/objective_tests/test_objectives.py	96.29% <100.00%> (ø)
...lml/tests/objective_tests/test_standard_metrics.py	100.00% <100.00%> (ø)

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[dsherry]

@gsheni if you check out #624 you'll see Angela added a "standard metrics" unit test file. Can you add something in there too? Other than that, looks great.

[dsherry]

@gsheni I took a quick look at the codecov report and didn't understand why it was failing for this PR. Feel free to ping #evalml-dev if that's still failing after your latest push and you'd like some help!