add rigid bed to GEOMETRY FILE in GAIA
After a discussion on the RER with Rebekka, we thought it would be nice that the user can directly put the elevation of the rigid bed in the geometry file. Now, one can specify it in the previous computation file, but this is less convenient for the user, because then one has to specify some extra obligatory variables. The idea is to add reading this variable already in the standard user_bed_init.f.