improve FrameRange/FrameSet string representations

[bcipriano]

The old C++ implementation has smart string representations - sequences of frames are grouped by range and step. I believe this only really affects visual representations of the frame range; the backend deals with only flattened lists of frames.

See the old libFileSequence test code[1] for examples of how the grouping works.

Our existing code uses very simple string representations - flat comma-separated lists of frames contained within. By Python and Java implementations should be switched to match the old C++ library.

[1] https://github.com/imageworks/OpenCue/blob/165d4c54b5e9ad55ec84416d8fdc79761174cabf/libFileSequence/test/FileSequenceTest.cpp#L800