Multiple inputs for Math Node

Being able to add as many inputs as we want in the math node would save a ton of nodes in some cases.

I realize some modes of the math node would not be compatible with more than two (or even one) input, but for the most commonly used ones it wouldn't be an issue and would make for much cleaner node trees :

• A + B + C + D... for Add
• A * B * C * D... for Multiply

(mockup mixing the group output node and the Add node, the socket shapes are messed up)