.add_derived_dt         Add derived temperature values for TMAX and
                        TMIN
.drop_source_vars_dt    Remove source vars if not requested
read_cru_dt             Create a data.table of CRU CL v. 2.0
                        climatology elements
read_cru_rast           Create a terra SpatRaster object of CRU CL v.
                        2.0 climatology elements
