Moved some multilabel stages to public repo. These stages convert the indexed probabilities predicted to named classes with probabilities.

[leahmcguire]

Related issues
Fixed metadata comparison and added error message about stage that failed when mleap serialization does not work.

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Additional context
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[TuanNguyen27]

LGTM!

[codecov]

Codecov Report

Merging #509 into master will increase coverage by 56.24%.
The diff coverage is 53.84%.

@@             Coverage Diff             @@
##           master     #509       +/-   ##
===========================================
+ Coverage   22.15%   78.40%   +56.24%     
===========================================
  Files         346      347        +1     
  Lines       11848    11859       +11     
  Branches      612      606        -6     
===========================================
+ Hits         2625     9298     +6673     
+ Misses       9223     2561     -6662     

Impacted Files	Coverage Δ
...orce/op/stages/impl/feature/MultiLabelJoiner.scala	40.00% <40.00%> (ø)
...ala/com/salesforce/op/stages/SparkStageParam.scala	54.38% <100.00%> (+0.81%)	⬆️
...a/com/salesforce/op/test/OpPipelineStageSpec.scala	97.95% <100.00%> (+97.95%)	⬆️
...ala/com/salesforce/op/features/types/package.scala	55.47% <0.00%> (+0.68%)	⬆️
.../stages/impl/feature/OpIndexToStringNoFilter.scala	6.25% <0.00%> (+6.25%)	⬆️
...la/com/salesforce/op/features/FeatureBuilder.scala	33.79% <0.00%> (+6.89%)	⬆️
...orce/op/aggregators/MonoidAggregatorDefaults.scala	92.85% <0.00%> (+7.14%)	⬆️
...ain/scala/com/salesforce/op/features/Feature.scala	33.33% <0.00%> (+8.33%)	⬆️
...scala/com/salesforce/op/features/types/Lists.scala	75.00% <0.00%> (+8.33%)	⬆️
... and 198 more

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[tovbinm]

please add license header in all the files