Print the optimal friction of a single turbine.
Add a new callback option "config.statewriter_callback", with which the user
can define a callback function which will be called at every timestep in the
forward shallow water equation with the pressure and velocity solution
as arguments. This can be used for performing diagnostics, saving the solutions
in custom file formats or other purposes.
Make checkpointing in OTF more robust:

1) When using the smooth turbine representation, hash the control values to avoid excessive storage usage.
2) Print a warning and continue if the checkpoint file was not found.