Add hacky MPI support for grid refinement

Works but is nowhere near anything one could consider good.
Obvious issues:

* More than one cuboid per grid makes it harder to determine the next lattice cell to be coupled
	* i.e. currently lattice positions are determined ad hoc by resolving their physical position
* Coupling is not actually parallelized
	* All coupling lines are traversed by all processes, way to much communication
* Load balancing and cuboid decomposition doesn't care about refinement
	* ideally refined cuboids should be computationally near their coarse _parent_ cuboids

The first two isses should be fixable with a reasonable amount of work.
This sadly doesn't apply in any form to the last issue.

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