all/male/female output in `stratum_averaged_weight` are not averages

[leewujung]

Currently the outputs from stratum_averaged_weight are not "averaged weights" per se since the numbers for male and female add up to be the numbers for "all" fish, which does not make sense.

Looks like the current outputs are a "weighted" versions of the male averaged weight and female averaged weight.

We should absorb this weighting into the downstream operations so that stratum_averaged_weight actually outputs averaged weight for all, male, and female fish.

I'll trace down the computation details in the current implementation and work with @brandynlucca to make changes.

[leewujung]

In addition, has the input argument group_columns ever been used for columns other than “stratum_ks”. If not, we can just rename this.