More imputation options

[shaycrk]

In the future, we'll likely want to add additional imputation options (e.g., within-group mean, most frequent category for categoricals, most frequent pattern for categoricals, multiple imputation, etc.), but this will take a bit of extra work beyond the current imputation code.

The current version simply provides a COALESCE snippet to drop in for a column definition, while these additional options will generally require additional joins, handling the categoricals as a group, or even pulling the data out of postgres for model-based imputations.

[jesteria]

This issue was moved to dssg/triage#309