Refactor transformers so that they are consistent

[jdreo]

Current implementation of transformers in transformers.py shows the technical debt of the history of the search for the best architecture. This should be refactored, as:

• Put the value making code within a ValueMaker.
• Re-use the parent's Transformer.__call__ function as much as possible.
• Make parameter names prefixed by the transformer name (e.g., split_separator), to allow composition.