Create convenience constructor for the Transforms (workitem related to #371)

[zeahmed]

This work item is related to the new API proposal #371

Following is the initial list of transforms that will be implemented to gather feedback.

• BootstrapSampleTransform
• CategoricalHashTransform
• CategoricalTransform
• ConcatTransform
• CopyColumnsTransform
• CountFeatureSelection
• DropColumnsTransform
• LpNormNormalizerTransform
• NAFilter
• NormalizeTransform

There are different ways to create a public facing API for these transforms based on their class definition.

Add Convenience constructor directly to the transform class.

It is only possible for following type of transforms.

• BootstrapSampleTransform
• ConcatTransform
• NAFilter

Add Create method (a helper method) for transforms that have Create method already implemented.

This is possible for following transforms.

• CategoricalHashTransform
• CategoricalTransform
• CopyColumnsTransform
• CountFeatureSelection

Add CreateX method (a helper method) for all possible use case of a transform where X is the name of the operation.

This is because one transform can instantiated for multiple purpose based on the argument e.g.

• DropColumnTransform (can be instantiated for dropping or keeping columns)
• LpNormNormalizerTransform (can be instantiated for LpNorm or GCNorm)
• NormalizerTransform (can be instantiated for MinMax, Mean-Var, LogMean etc.)

[zeahmed]

@TomFinley, can you please comment on what would be the best way to move forward for all the three cases above?

[TomFinley]

HI @zeahmed , I think I generally like that plan at least to start. I feel like direct construction where possible is the best option, because that makes sense: you're instantiating a transform. The Create semantics are nice where possible as well. (Meta-transforms are what I see in this list, which makes sense.) And in cases where a transform can be instantiated with multiple argument classes (e.g., normalizers), then your plan for CreateX makes sense.

Just my thoughts.