Output mass fluxes into averages NetCDF file

[arango]

I added the capability to save mass fluxes (DU_avg1, DU_avg2, DV_avg1, DV_avg2) for coupling 3D momentum into the time-averaged NetCDF file. In the code these fields are averaged over all the barotropic time-steps. Here, they are also averaged over the specified baroclinic steps. This is only done for output purpuses (AVERAGES and FORWARD_WRITE) so the average NetCDF can be used as the basic state to linearize the tangent linear and adjoint models. This is only relevant in adaptive sampling and optimal observations.