Identify the data matrix containing a given response variable
get_resp_mat(resp, Mlvls, outnames)
character string; the name(s) of the data matrix/matrices of the response variable(s)
character string; name of the response variable
named vector where the names are all column names of all data matrices, and the values are the names of the corresponding data matrices
character vector; names of the columns in the data matrices that contain the response variable (or multiple columns in case of a survival outcome)