[Predator] Refactor model APIs to accept ``MetaFeature`` which returns a list ``FeatureValue``
There are 2 directions:
1. Add ``MetaFeature`` and ``MetaWeight`` for it, we treat them the same as normal ``Feature`` and ``weight``.
2. We can also have each Feature also be X -> Y -> list(FeatureValue), where the non-meta features always return a singleton list
the tricky bit is getting the weights to match up; i.e., the dictionary passed into the LogLinearChangelistClassifier constructor
Comment 1 by kateso...@chromium.org
, Jan 18 2017