Landfast bathymetry implementation

[apcraig]

See #573. We want to move away from the hardcoded 40 level implementation and rethink how that is implemented. One other issue is that even if bathymetry and landuse is off, it still reads in the hardcoded 40 levels and blows out of bounds if max(kmt)>40. This is the case in RASM and is why #593 was needed in the mean time.

[TillRasmussen]

I would force the code to read the depth. This can either be done through the existing variable kmt or by creating a new variable, which could be Bathymetry.
One exception would be if someone passes the depth through an coupling interface.