nestor causes crash when dredging deeper than non-erodible layer
We recently had a crash using Nestor. Apparently, the reason was that dredging occurred in nestor to a depth, that was deeper than the elevation of the rigid bed (so you can say that our input was wrong). Nevertheless, I think it would be good t add a check for this in Nestor. Note that we noticed the issue in v8p1. I still need top check whether the issue is stil there in the latest version.