Add explanation to selection of std and mean

core/src/main/scala/com/salesforce/op/stages/impl/feature/OpScalarStandardScaler.scala

@@ -62,13 +62,13 @@ class OpScalarStandardScaler
 
  val std = scalerModel.std.toArray
  val mean = scalerModel.mean.toArray
- if (std.length == 1) {
-  val stdMean = std.head
-  val meanMean = mean.head
-  val scalingArgs = LinearScalerArgs(1 / stdMean, - meanMean / stdMean)
-  val meta = ScalerMetadata(ScalingType.Linear, scalingArgs).toMetadata()
-  setMetadata(meta)
- }
+
+ // Since is a UnaryEstimator, exactly one value will be in std and mean
+ val stdVal = std.head
+ val meanVal = mean.head
+ val scalingArgs = LinearScalerArgs(1 / stdVal, - meanVal / stdVal)
+ val meta = ScalerMetadata(ScalingType.Linear, scalingArgs).toMetadata()
+ setMetadata(meta)
 
  new OpScalarStandardScalerModel(
  std = std,