Small adjoint bug and few typos

[arango]

The routine step3d_uv.F assigns the value of local variable AK(i,N(ng)) twice. This reassigment is redundant in the nonlinear, tangent linear, and representer model and doesn't affect the results. However, this constitutes a bug in the adjoint model since reassigments are added twice.

There is also few typos associated with 4DVAR adjustement of surface forcing fields in routines ad_wrt_his.F, tl_wrt_his.F, and cost_grad.F. Many thanks to Gregoire Broquet for reporting these typos.

The routine step2d_LF_AM3.h has an unnecessary specification of I_RANGE and J_RANGE when updating rzeta. This is not a bug. Similar cleaning is done in the TLM, RPM, and ADM versions of this routine.

Corrected a bug in nemuro.h when computing zooplankton egestion. Many thanks to Al Hermann for reporting this bug.