Error on fixed dt leading to negative storage

[visr]

We use isoutofdomain to reject timesteps leading to negative storage. I think this only really works for adaptive timestepping, where it can reduce the dt. For fixed dt it is still possible to get negative storage if you have a Basin with a very large outflow, which steps over our smoothing functions. We should also do this check ourselves in a FunctionCallingCallback, with an informative error message pointing to the Basin node(s) that has negative storage.