Introduces the 'FeatureBlock' as the main interface for Triage to create features, instead of directly interfacing with collate.

- Add FeatureBlock abstract base class. The abstract methods are aimed
at providing an easy, flexible interface for feature generators to
implement. The concrete methods are aimed at building on those abstract
methods to provide a slightly easier, higher-level interface for
Experiments, et al to use.

- Splits up the bloated architect.FeatureGenerator into a couple
different places: the code that generates is now in SpacetimeAggregations in feature_block _generators, and the code that runs the generated queries now lives in concrete methods of the FeatureBlock base class.

- ExperimentBase now passes on config to generate_feature_blocks and
owns a collection of resulting FeatureBlocks, instead of a
FeatureGenerator object.

- Instead of a FeatureDictionaryCreator object, the FeatureDictionary
class is introduced that knows how to build itself from a collection of
FeatureBlocks. The Experiment holds references to these as before.

- Continuing the trend of molding collate to more closely fit how Triage
is using it and removing unneeded flexibility, the unused Aggregation base class is folded
into the SpacetimeAggregation so the latter can directly inherit from
FeatureBlock. This way we get to remove/condense many tests, and sunset
the collate integration tests because the methods being tested no longer
exist and are tested in either SpacetimeAggregation or FeatureBlock. In
addition, many of the arguments within SpacetimeAggregation are changed
to their FeatureBlock equivalents to more tightly fit as a FeatureBlock
subclass. Some of the more generic helper methods within
SpacetimeAggregation are moved to FeatureBlock as concrete methods

- With feature data now residing at a new config key, the experiment's
config version is bumped to v7.

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