Alternative scoring metrics

[carl-offerfit]

• Allow specification of an alternative sklearn score function for double ML models
• Add score_nuisances function

[carl-offerfit]

@kbattocchi Here's an update:

1. I fixed the type hints to use the old Union approach rather than the python 3.10 |. That should fix the Python 3.8 & 3.9 errors.
2. I added a unit test for the ML score signature validation
3. I researched sklearn make_scorer and as far as I can tell it doesn't do any validation at all - if the function you passed to make_scorer is not valid it just fails when you try to use it. That makes me wonder if my approach was over engineered, and instead we should just let it fail. At the same time, I'm a bit attached to the approach since I spent time on it. ;-) WDYT?