Upload multiple covariates

Presumably this would also be useful to users and should be more straightforward than multiple outcomes. Just need to allow multiple `covar.` columns in the `setup_load` and then select the covariate to use before running `covariate_model`.

Trace the setup_load and covariate_model entry points to understand how covariate columns are loaded and selected before modeling. Check how multiple outcomes are handled for a comparable flow. Done means multiple covar. columns can be uploaded, one covariate can be selected, and covariate_model runs with the chosen column.

Written by the indexing model from the issue text.