Added concrete wrappers for HashingTF, NGram and StopWordsRemover

[tovbinm]

Related issues
These stages fail to serialize with changes in #274

Describe the proposed solution
Added concrete wrapping stages as for the rest of the stages + tests of course ;)

Describe alternatives you've considered
NA

[codecov]

Codecov Report

Merging #314 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
+ Coverage   86.48%   86.51%   +0.03%     
==========================================
  Files         322      325       +3     
  Lines       10579    10581       +2     
  Branches      343      565     +222     
==========================================
+ Hits         9149     9154       +5     
+ Misses       1430     1427       -3

Impacted Files	Coverage Δ
...force/op/stages/impl/feature/OpIndexToString.scala	100% <ø> (ø)	⬆️
...force/op/stages/impl/feature/OpStringIndexer.scala	100% <ø> (ø)	⬆️
...alesforce/op/stages/impl/feature/OpHashingTF.scala	100% <100%> (ø)
.../scala/com/salesforce/op/dsl/RichListFeature.scala	100% <100%> (+14.28%)	⬆️
...om/salesforce/op/stages/impl/feature/OpNGram.scala	100% <100%> (ø)
...ce/op/stages/impl/feature/OpStopWordsRemover.scala	100% <100%> (ø)

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[wsuchy]

LGTM