d2o: _contraction_helper does not work when using numpy keyword arguments

The _contraction_helper passes keyword arguments to the underlying numpy functions (axis=, keepdims=). The result of the _contraction_helper's local computation is then an array and not a scalar. Therefore the dtype check fails.

Fix: After solving theos/NIFTy#2, adopt to the case that the local run's result object is an array and make a further distinction of cases, i.e for something like axis=0 for the slicing_distributor.