This function is intended to be mapped over a sequence i
inside the
function coalesce_many
coalesce_many_helper(df, new_split_names, i)
returns a data frame with one set of paired columns combined into a
single column and named based on new_split_names
a data frame following a join and thereby containing paired columns of the form Col_1.x, Col_1.y
a list of desired column names, e.g. Col_1, Col_2
a number between 1 and the length of new_split_names
coalesce_many_helper
runs inside
coalesce_many