Final public API for remaining learners

[Zruty0]

Part of #1698

Let's make good public classes for trained artifacts of:

• FFM

• Naive Bayes

• PCAAnomaly

• MF

• Any other trainers not covered in sub-issues of Final public API for predictor classes #1698

• Move to an appropriate namespace inside Microsoft.ML

• Reduce the public surface

• Add public constructor

• Add a sample

[singlis]

NOTE: public constructor should include consistent parameter usage and order (i.e. we have a pattern of LabelColumn, then FeaturesColumn). See #1806.

[najeeb-kazmi]

NOTE: public constructor should include consistent parameter usage and order (i.e. we have a pattern of LabelColumn, then FeaturesColumn). See #1806.

@singlis The LabelColumn, FeaturesColumn pattern is in trainers. These work items are related to predictors instead, which I am renaming from Predictor to ModelParameters. So far, I have not seen instances of predictor constructors taking in label column or feature column, but if I do encounter one, I will stick to the convention that you describe.