Use multi input socket on commutative math operations

For the math node, on operations such as addition and multiplication, add a multi input socket. This will really help simplify large nodetrees.

The first socket can be a regular socket, but the second one can use a multi input socket. That way, subtraction, powers, and other non-commutative operations can first add all of the second socket's values, then for instance divide the first value by the second. This will make the node type somewhat interchangeable.