Use `group` on `tbl` for aggregations

[edwindj]

A typical use case is to impute per group (e.g. with a mean).

A useful addition would be that dcmodifydb is able to use the grouping on the table to do the aggregations.
Downside is that that is not explicit in the rule (which would be better).

[edwindj]

I think those can be handled with do_by, mean_by as used in validatedb