Add AcyclicTransformer helper

[dsnet]

Add AcyclicTransformer helper, which wraps Transformer with a FilterPath
that ensures that the transformer cannot be recursively applied upon
the output of itself. Adjust documentation of Transformer to suggest usage
of AcyclicTransformer if necessary.

Fixes #77

[dsnet]

@neild, did you have any suggestions on the name? Or does AcyclicTransformer SGTY?

[neild]

AcyclicTransformer SGTM. It's a mouthful and not really explanatory enough to understand what it does absent the documentation, but strikes a decent balance between being too verbose and meaningless. I can't think of anything better.