Clear and structured support for any optimization metric(s) about gama

[PGijsbers]

Resolved in 0de5e1b. Only accepts custom metrics through creating your own Metric. I will probably keep it that way. Before closing this issue it needs to be decided if this is the way custom metrics will be supported. If so, an example needs to be added to the documentation before this issue gets closed.

[PGijsbers]

The score function needs to be adapted to return multiple scores if more than one optimization score was given, or needs to clearly state it will only return one.

[PGijsbers]

With mixed metrics, currently only predict_proba is called, and the class predictions it taken to be the class with highest probability. Instead, predict should also be called and those predictions should be used for class-label metrics.