Corrected bug with the mass point sources, Q_PSOURCE

[arango]

The option Q_PSOURCE was corrected in step3d_t.F so it is only activated to the desired tracers using:

             DO is=1,Nsrc
               IF (LtracerSrc(itrc,ng)) THEN
                 ...
                 DO k=1,N(ng)-1
                    FC(i,k)=FC(i,k)+0.5_r8*                             &
     &                      (Qsrc(is,k  )*Tsrc(is,k  ,itrc)+            &
     &                       Qsrc(is,k+1)*Tsrc(is,k+1,itrc))
                 END DO
               END IF
             END DO

Notice also that the k-loop range was corrected. We need to have k=1,N(ng)-1. This update to the FC flux is only at the interior since it defined at W-points and Qsrc is only possible at RHO-points.

Many thanks to Lyon Lanerolle for bringing this to my ​attention.