Make Feature class public instead hide it's ctor

[tovbinm]

Related issues
Impossible to change the internal feature fields after creation, e.g response/predictor.

Describe the proposed solution
Making Feature case class public to allow changing feature fields if needed using .copy method. For example if one would like to change the type of the feature from response to predictor.
We still maintain the ctor as private, enforcing usage of creating features through FeatureBuilder.

Describe alternatives you've considered
None

[codecov]

Codecov Report

Merging #45 into master will increase coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #45      +/-   ##
=========================================
+ Coverage   83.47%   83.9%   +0.42%     
=========================================
  Files         296     296              
  Lines        9380    9679     +299     
  Branches      344     544     +200     
=========================================
+ Hits         7830    8121     +291     
- Misses       1550    1558       +8

Impacted Files	Coverage Δ
...ain/scala/com/salesforce/op/features/Feature.scala	40% <ø> (ø)	⬆️
...cala/com/salesforce/op/utils/io/csv/CSVInOut.scala	0% <0%> (-100%)	⬇️
...es/src/main/scala/com/salesforce/op/OpParams.scala	89.58% <0%> (+4.16%)	⬆️
...a/com/salesforce/op/testkit/PartiallyDefined.scala	100% <0%> (+100%)	⬆️
...om/salesforce/op/testkit/FeatureFactoryOwner.scala	100% <0%> (+100%)	⬆️
...in/scala/com/salesforce/op/testkit/RandomSet.scala	66.66% <0%> (+66.66%)	⬆️
...com/salesforce/op/testkit/StandardRandomData.scala	100% <0%> (+100%)	⬆️
...ala/com/salesforce/op/testkit/InfiniteStream.scala	100% <0%> (+100%)	⬆️
...n/scala/com/salesforce/op/testkit/RandomData.scala	100% <0%> (+100%)	⬆️
...ala/com/salesforce/op/testkit/RandomIntegral.scala	100% <0%> (+100%)	⬆️
... and 8 more

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[leahmcguire]

should we make FeatureLike private to avoid confusion?