Replace assertion by test and DUNE_THROW

For the case that the targetProcessors input array of the loadBalance method
does not have the correct size, throw an exception rather than having an
assertion fail.
Use a MCMGMapper to assign indices to elements in the loadBalance method

Otherwise you will get wrong results when loadbalancing grids with more than
one element type.
Ignore requested ranks for Ghost entities.

It is not clear what UG does with them, so we better just don't pass them
on to begin with.