remove vals in 2 list components with the same name

From 2 list components with the same name, remove vals that aren't in the compare object

Usage

rm_vals(return_obj, compare_obj, name_in_obj, colnames)

return_obj: the object (containing list component of name in obj) that will be modified. Only combinations of the columns found in compare object will be retained
compare_obj: the object (containing list component of name_in_obj) that return_obj will be compared to
name_in_obj: the name of the list elements to use; the same name must be in return_obj and compare_obj. This list element must be a data frame with the same column names
colnames: The column names within the name_in_obj list components to compare.

return_obj[[name_in_obj]], modified to only include elements present in compare_obj[[name_in_obj]].

Kathryn Doering