Quentin 10/01/2019: add a local filtering for very high slopes in PGD step (for a better chance of pressure solver convergence). In namelist NAM_ZSFILTER : possibility to activate or not (LHSLOP), choose the number of filtering iterations (NLOCZSFILTER) and the slope value (XHSLOP= Dz/Dx and Dz/Dy) where the filter is applied