Write basic feature stats to tables after feature generation

@saleiro @ecsalomon and I went on a bit of a dive this morning following my exploration of the https://github.com/tdda/tdda repository . The team has talked about "premodeling" before, that could expand in scope to be as big as postmodeling. Right now, however, I think we should figure out whats the simplest possible thing that could be useful as a beachhead for this premodeling. And in our discussion I think we have something.

After feature generation (both pre-imputation and post-imputation), create feature stats in table/s that encompass basic min/max/nulls stats for the created features. These would always reflect the current state of the entire feature table (e.g. we recompute each time), provided that this doesn't take too long.