Use Multiprocessing besides domain decomposition in solve function

[steffenger]

Opening this as per the discussion in #68.

Maybe a user wants to have a certain number of micro simulations on a MPI process, so that the local adaptivity can still be used to avoid memory requirements instead of having one MPI process for every micro simulation. In this case one could benefit from multiprocessing in the solve function on the local domain.

[uekerman]

This would require thread safety from the micro simulation software, what we probably cannot expect in general. E.g. the micro simulation software could not use static variables in crucial places.