add hashing for MPI equality check for fields in optimize_kl

Merged Jakob Knollmueller requested to merge fix_mpi_bug into NIFTy_8

checking MPI equality of large fields explicitly can crash and lead to segmentation faults, that's why we introduced the possibility to just compare hashes. We didn't add it to the check in the _single_value_sample_list in and I run into an issue there.

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