Dev pressurefix; horizontal pressure gradient

[StevenvdLinden]

1. Added new option for horizontal pressure gradient (to enable pressure-driven channel flow). Also supports time-dependent pressure gradient.
2. lcoriol and lpressgrad switches are now mutually exclusive, fixing implicit behavior that pressure gradient would always be set to zero, when lcoriol=.false. (see modforces.f90, lines 370-379).
3. Input infrastructure remains the same. Columns in lscale.inp.iii are read in as either geostrophic winds (when lcoriol=.true. or as horizontal pressure gradients when lpressgrad=.true.. Same for netcdf input.
4. Error is produced when both are set to true.

Background: the old switch lpressgrad would have allowed to impose no pressure gradients, whilst still allowing Coriolis forces. This option is now made explicit: you must put geostrophic winds to zero when you want this to happen (as intuitively most of us would have already done anyway)