make regression metrics 'multioutput' behavior consistent with scikit-learn (fixes #818)

[jameslamb]

Fixes #818.
Fixes #819.

Changes in this PR

• ensures that regression metrics respect compute argument when multioutput == "raw_values"
• ensures that regression metrics' treatment of multioutput=None matches the behavior of their scikit-learn equivalents
• removes irrelevant comment about np.average() in regression metrics
• adds tests on behavior of metrics for different values of multioutput

Background

See the linked issues for an explanation. I believe this PR is worth accepting because it will resolve an inconsistency with scikit-learn and with dask-ml itself, but it is technically a breaking change.

Thanks for your time and consideration.

[TomAugspurger]

Agreed that the old behavior was incorrect. Thanks for the fix.