Add scoring parameter for SplineCV

[JamesSample]

I've added scoring as an additional parameter in SplineCV to control the scoring metric used during cross-validation. I've also added a simple test to test_spline.py, which checks that results from SplineCV using the new scoring parameter match those from standard Spline combined with cross_val_score. I'm not sure I've put this test in the right place - please let me know if it should be elsewhere.

Relevant issues/PRs:

(Hopefully) fixes #379 (see also the discussion here).

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[leouieda]

Thank you for adding this! Made a few minor suggestions and a question at the end about the test. Otherwise, this looks very good to me. Just waiting for the CI to finish running tests and checks to see if there is anything else that needs addressing.

[leouieda]

Looks great! Merging this in 🚀

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[leouieda]

@JamesSample please see above ☝🏾 about adding yourself as an author 🙂 Thanks for taking the time to report and implement this!