Deterministic MPI KL

Merged Martin Reinecke requested to merge deterministic_mpi_kl into NIFTy_6

This patch tries to carry out summations over samples in a completely deterministic way, independent of the number of MPI tasks involved. My first tests with EHT indicate that this works, i.e. we get bit-identical results for the same initial conditions when varying the number of tasks.

@reimar I'm not absolutely sure that we have a test for _metric_sample. How hard would it be to add one?

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