Implement partition for coarsening

[holke]

If the appropriate flag is set in forest partitioning, families should not get seperated by process boundaries, thus ensuring that coarsening one level is a process local operation.

This needs changing the first and last elements on a process such that only whole families land on a single process after the partition operation.

[lukasdreyer]

@holke
Do you have an idea how to compute the new offsets after computing the standard offsets, without sending the elements first?
All my ideas boil down to essentially this (with different possibilities on how to do the adjustements):

Do the computation of the standard offsets, send the elements, then adjust the number of elements on a process slightly (considering edgecases like empty ranks), compute new offsets and finally send some elements again according to these new offsets.

[holke]

Have not thought about this yet.