Rewrite column name handling to use constants

[frewmack]

Currently, we keep track of the name of each column that the user specifies in their uploaded GWAS data, and use that string to get the respective column in the GWAS dataframe. This is unnecessary, since we can instead identify each column based on what the user entered in the form (eg. BP column is called "BP_hg38" by user, etc.), and then rename the column label in the dataframe to use a reusable constant.