Easy sub for DV in nm_input

[kylebaron]

We frequently have a DV and LNDV column in our data sets. When calling nm_input(), we have the opportunity to rename columns, but it is a couple of steps given NONMEM's rules.

I wonder about

nm_input(spec, new_dv = "LNDV")

which would both drop the DV column and set DV = LNDV.