Implement separate snapshot output resolution for x- and y-dimensions

[martijnende]

The spatial resolution of the snapshot output (OX) is controlled by NXOUT, but this only applies to the x-coordinate. The output resolution in the y-direction is the mesh resolution. It would be better to also have a NYOUT. Then, instead of:

do ixout=1, nn, pb%ox%nxout
    ...

one should have:

do ixout=1, nx, pb%ox%nxout
    do iyout=1, ny, pb%ox%nyout
        nout = (ixout - 1) * ny + iyout
        ...

in the OX module of output.f90 (currently being revised in #43)