Add ability to save/load the problem state to/from disk
Now that we have the Equations interface, we should think about some general way to save the solution (+ parameters etc.) to the disk, so that they can be restored later on.
Some things to discuss:
- Should the unknowns be saved on the Problem-level or on an Equation-level, i.e., Problem.u or
for eq in Problem.equations: eq.save(...)
? - The parameters need to be organized somehow in order to be stored/reloaded
- When loading, we should do a check, whether the number of equations/dofs of the current Problem and the saved state align