Allow SDK use of ai.deepsense.deeplang.doperables.Transformer

[metasim]

Transformer and the associated TransformerAsOperation classes serve a useful role in abstracting over the construction of transformers. SDK users can not build against them because the abstract method Transformer._transform is marked as private[deeplang]. Would the project consider agging it and _transformSchema as protected instead?

[metasim]

I'm happy to submit a PR for this if amenable.

[jaroslaw-osmanski]

Please submit a PR. It will be accepted.

[metasim]

Turns out that just changing the private[deeplang] to protected doesn't work, because TransformerWrapper and StringIndexingWrapperModel call the _transform and _transformSchema methods directly on the entities they wrap. As best I know, there's no way in Scala to say access is either protected or private[xyz].

[metasim]

Addressed in #36