Luv colorspace conversions

Adding XYZ <--> Luv, RGB <-->
Adding test functions to mirror Lab tests
Adding references regarding Lab and Luv spaces to docstring
Removing unneeded division checking code
renaming machineEps to eps to match convention
flake8-compliant formatting
Cleaning up docstrings
docstring changes to correctly format input/output, clarifying XYZ and reference white
Adding note for Luv colorspace to top docstring
Format color conversion docstrings.
Merge pull request #1 from stefanv/luv-colorspace

Format color conversion docstrings.
Merge pull request #798 from matttrent/luv-colorspace

Add Luv colorspace