Corrected bug in the adjoint of the NPZD models

[arango]

A small bug was corrected in ad_npzd_Franks.h, ad_npzd_powell.h, and ad_npzd_iron.h when updating the global biological tracers:

We need to have:

              ad_cff=ad_cff+Hz(i,j,k)*ad_t(i,j,k,nnew,ibio)
instead of
              ad_cff=add_cff+Hz(i,j,k)*ad_t(i,j,k,nnew,ibio)

We need to use ad_cff and not add_cff. Many thanks to Ha Joon Song for reporting this ​typo.