Better errors when things don't work because a model has not been solved/initialized

[dbrakenhoff]

I already added a model.initialized attribute that should track whether a model was initialized or not. That attribute can be used to check model status. Maybe we need something similar for the solve?

This issue is meant to improve error messages when certain things do not work because a model is not solved/initialized.

• I ran into some plotting difficulties with cross section models that were not initialized. It would be good to check what plotting actions work without initialization, and especially for the overview plots it would be nice to allow direct plotting, without initialization first.
• heads, velocities, disvecs etc. should all give an informative error when a model was not solved. I think we should be strict about this even for models that have no unknowns. Users that accidentally forget to solve should not be confronted with some error about ngbvc, but something informative to them :)
• ...

Perhaps we can implement it as a decorator to save code:

@model_solved()
def head(self, x, y, t, layers=None):
    ...