TransformationSet -> ImageSet, transform -> image
TransformationSet -> ImageSet in printing/docs
deprecate TransformationSet
add fields to deprecated flag on TransformationSet
Change feature keyword in TransformationSet deprecation
add imageset
imageset of Intervals that cross 0
fix import in test_fancysets
add import to imageset docstring
_image -> _eval_imageset
Merge pull request #2341 from mrocklin/image-set

Replace TransformationSet term with ImageSet
First push with basic functions
Added gradient, rectified mistake in scalar_potential
Added tests, temporary helpers