Return scoring feature distributions from RawFeatureFilter

[tovbinm]

Related issues
Scoring feature distributions are not exposed from RawFeatureFilter

Describe the proposed solution
Return training and scoring feature distributions in single collection from RawFeatureFilter, but add a type to distinguish between them.

• Added type enum field for FeatureDistribution with Training and Scoring possible values.
• Made sure OpWorkflow can read / write old feature distributions (without the type).
• Added tests
• Quite some avro logging which started with this commit Using MapReduce Api for Avro Read Write #150

Describe alternatives you've considered
Add an alternative collection with scoring feature distributions everywhere in RawFeatureFilter, OpWorkflow etc.

Additional context
This is an important piece of information that we don't want to loose ;)

[codecov]

Codecov Report

Merging #171 into master will increase coverage by 0.02%.
The diff coverage is 91.52%.

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   86.22%   86.25%   +0.02%     
==========================================
  Files         305      305              
  Lines        9937     9950      +13     
  Branches      345      531     +186     
==========================================
+ Hits         8568     8582      +14     
+ Misses       1369     1368       -1

Impacted Files	Coverage Δ
...la/com/salesforce/op/features/types/OPVector.scala	100% <ø> (ø)	⬆️
...scala/com/salesforce/op/features/FeatureLike.scala	42.39% <0%> (+0.45%)	⬆️
...a/com/salesforce/op/filters/PreparedFeatures.scala	80.48% <100%> (ø)	⬆️
...om/salesforce/op/filters/FeatureDistribution.scala	98.33% <100%> (+0.33%)	⬆️
...rce/op/stages/OpPipelineStageReadWriteShared.scala	100% <100%> (ø)	⬆️
...a/com/salesforce/op/filters/RawFeatureFilter.scala	89.28% <100%> (+0.29%)	⬆️
...cala/com/salesforce/op/OpWorkflowModelReader.scala	91.3% <100%> (ø)	⬆️
.../main/scala/com/salesforce/op/OpWorkflowCore.scala	93.54% <100%> (+0.21%)	⬆️
...cala/com/salesforce/op/OpWorkflowModelWriter.scala	100% <100%> (ø)	⬆️
.../src/main/scala/com/salesforce/op/OpWorkflow.scala	87.5% <90%> (-0.09%)	⬇️

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[Jauntbox]

LGTM