Support new user ratings across LensKit

[mdekstrand]

We need to support new user ratings across LensKit, at least for every algorithm where it makes sense.

• [x] Bias
• [x] Item-Item
• [x] UnratedItemsCandidateSelector
• [x] Popular (indirectly supported through the candidate selector)
• [x] ALS Explicit (#114)
• [x] ALS Implicit (#114)
• [ ] #684
• [x] FunkSVD - we probably don't actually want to do this, just document that it isn't supported and raise a warning