[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 params.input file.

Also the names of the borders should be changed to be better understandable.