[wip] add repeating and spanning basis functions

[RensDimmendaal]

I'm building preprocessing functions to address:
#20
#29

Open Issues:

1. RepeatingBasisFeatures currently accepts only one feature. This is problematic for most tests.

[kayhoogland]

Any updates on this @RensDimmendaal? Would love to use it!

[RensDimmendaal]

Far point. I have left this to sit for too long. It is unclear to me how to run the standard tests this because transformer only accepts one column. @koaning mentioned the idea of wrapping the transformer with a column selector for the tests. I'll try that in the next 7 days.

[MBrouns]

Wrapping the thing in a columntransformer is probably a reasonable default anyway! Basically something like this

class RepeatingBasisFunction:
    def __init__(column, *args, **kwargs):
        return ColumnTransformer([
            ('repeatingbasis', _RepeatingBasisFunction(*args, **kwargs), column)
        ], remainder='passthrough')

class _RepeatingBasisFunction:
    pass # Your current implementation

[RensDimmendaal]

Continued in a different branch #171