Move tracking valid imputations to collate

_From @shaycrk on October 2, 2017 19:26_

Make the collate imputation objects keep track of their parameters and types of columns they can operate on so `FeatureGenerator._validate_imputation_rule()` doesn't need to be explicitly updated with every change to the supported imputation types.

_Copied from original issue: dssg/architect#47_

Start by locating FeatureGenerator._validate_imputation_rule() and the collate imputation objects, then trace how supported imputation types and column parameters are currently represented. The refactor is done when validation derives this information from the collate objects and no longer needs explicit updates for each supported type.

Written by the indexing model from the issue text.