Reduce the number of very similar `assay_var..` columns

[ppreshant]

There are 4 columns all of which serve the same purpose. Please clean them up into a single renamed column as mentioned in #1

> select(combined_data, contains('assay')) %>% colnames
[1] "assay_var.horz_label" "assay_variable"       "assay_var.identifier" "assay_var.label" ```