Use OS specific path separator

[tovbinm]

Related issues
#192

Describe the proposed solution
Use OS specific path separator

[codecov]

Codecov Report

Merging #193 into master will increase coverage by 6.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   80.33%   86.35%   +6.01%     
==========================================
  Files         309      309              
  Lines       10007    10009       +2     
  Branches      349      557     +208     
==========================================
+ Hits         8039     8643     +604     
+ Misses       1968     1366     -602

Impacted Files	Coverage Δ
...la/com/salesforce/op/utils/io/avro/AvroInOut.scala	100% <100%> (ø)	⬆️
.../salesforce/op/features/FeatureBuilderMacros.scala	0% <0%> (-100%)	⬇️
...sforce/op/stages/impl/feature/Transmogrifier.scala	96.74% <0%> (+0.29%)	⬆️
.../op/features/types/FeatureTypeSparkConverter.scala	98.19% <0%> (+0.9%)	⬆️
...n/scala/com/salesforce/op/dsl/RichMapFeature.scala	81.53% <0%> (+1.53%)	⬆️
...in/scala/com/salesforce/op/cli/gen/AvroField.scala	76.92% <0%> (+2.56%)	⬆️
...ges/impl/feature/DateToUnitCircleTransformer.scala	100% <0%> (+3.44%)	⬆️
...s/impl/feature/OPCollectionHashingVectorizer.scala	96.37% <0%> (+3.62%)	⬆️
...com/salesforce/op/utils/stages/FitStagesUtil.scala	94.73% <0%> (+3.94%)	⬆️
...scala/com/salesforce/op/testkit/RandomStream.scala	100% <0%> (+4.76%)	⬆️
... and 47 more

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