transformations

Problem
Some of our transformers & estimators are not thoroughly tested or not tested at all.

Solution
Use OpTransformerSpec and OpEstimatorSpec base test specs to provide tests for all existing transformers & estimators.

The Point#d, Vector#d and Direction#d modules were recently optimized in the coordinate-systems branch to avoid unnecessary object allocations and function calls, but this also meant most functions got more verbose and therefore more prone to typos. It would be very useful to add some more fuzz and/or hardcoded tests for the various functions in these modules, following the patterns in the