Switch to new numpy random generators

Merged Martin Reinecke requested to merge new_random into NIFTy_6

After the change some test fail. I'm not absolutely sure whether they fail because they rely on a specific seed. If so, we should change them so that this is no longer the case.

@reimar The MPI KL still needs a fix to work consistently even when the number of tasks is changed. I added a FIXME comment. If it's easy for you to implement, please go ahead, otherwise I'll try to do it at some point.

Still TODO: docstrings in nifty6.random.

@parras, @reimar: What do you think about the interface?

Edited by Martin Reinecke

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