Introduce 'out' parameter for operations.__binary_op

[Markus-Goetz]

Feature functionality
Allow the generalized binary op to accept and additional output tensor as parameter in which the result is written. If not given (i.e. =None), then the current behavior is assumed, instantiating a new object.

Unsure about assignee, could be both a binary_op topic (Charlie) or generalized operations topic (Claudia).