Raise warning when self.classes is an empty list for voters.

[hhelm10]

Is your feature request related to a problem? Please describe.
When manually using voters I did not realize I had to give the set of classes and that otherwise is was using an empty list.

Describe the solution you'd like
Raise a warning when self.classes is empty and you try to make a prediction.

[levinwil]

@hhelm10 did you use LifelongClassification{Forest, Network} or the raw progressive learner class?

[hhelm10]

This was when using a TreeTransformer / TreeVoter directly. Probably an edge case, but adding a warning is pretty harmless.

[levinwil]

Gotcha. I just wanted to make sure it didn't show up when using LifelongClassification{Forest, Network}