Add in feature subsetting

[thcrock]

We need two classes, that can be configured via the experiment config (names subject to change):

FeatureGroup

FeatureSubsettingStrategy

These are configurable per experiment, and thus their definition must be YAML-serializable. The former defines a way to create feature groups. Whether it be by table name, time, aggregation, etc. To start we are just implementing table names. The class will take in its configuration in the constructor, and have a method that takes in a feature dictionary and outputs the subset of that feature dictionary.

Lists of groups are paired with a strategy (leave one out/leave one in/use all combinations). The strategy, given a list of groups, can output an iterator/list of feature dictionaries.