Allow user to flexibly specify weights

[erika-redding]

No description provided.

[ashleyasmus]

Might be good to use SANDAG weighted data as a test case. It has like six different weights ! !

[ashleyasmus]

I was looking into this, and it seems to all stem from the hard coded 'id' and 'weight' column names in hts_get_keycols. cursed! We should allow the user to specify the wtname, the idname ~ somewhere in prep data (??) and then carry that all the way through.

unfortunately hts_get_keycols is called by many other functions, which are in turn called by many other fucntions, inside the family of 'prep data' stuff