[BUG] [CV] ClassPerformance can't accept list in alternative_scorers

[noamzbr]

Describe the bug
The parameter alternative_scorers of ClassPerformance is documented as needing to be a list (and is tested to be a list) but the actual code using it assumes it's a dict {name: metric}, like the outputs of get_default_classification_scorers for e.g.

To Reproduce
Run ClassPerformance with alternative_scorers=[SomeMetric]

Expected behavior
alternative_scorers should work, with one option only (probably list)