d2o: Contraction functions rely on non-degeneracy of distribution strategy

Several methods of the distributed_data_object rely on the fact, that the distribution strategy behaves as if the local data was non-degenerate. Currently the non-distributor fixes this by returning trivial (local) results in the _allgather and the _Allreduce_sum method.

Affected d2o methods are at least: _contraction_helper, mean

Fix: Move the functionality for sum, prod, etc... into the distributor.