[shallowwater] Make the borders of the LET limiting changeable
The LET limiting is used to limit the flux for small water depth. The limiting starts if the
waterDepth is lower than
upperH. The second border is
lowerH. For all
waterDepth lower than
lowerH the flux is set to zero.
Currently the borders are hard coded. This should be changed. Instead they should be changeable via the
Also the names of the borders should be changed to be better understandable.