Add split value argument to ColSplitter

[DanteOz]

Adds an optional parameter to ColSplitter to specify which value to split on. Done with intention of simplifying the use of a k-fold validiation index column with the DataBlocks API. Example usage would be:

df = pd.DataFrame({
    'a': [0,1,2,3,4,5], 
    'fold': [1,2,3,1,2,3]
})
splits = ColSplitter('fold', 3)(df)

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[DanteOz]

@jph00 Ready for review. I added support for splitting on a list of values, given the discord discussion with Benjamin.

[jph00]

Good stuff!