Allow math node to work with variadic inputs

[optozorax]

Requires variadic inputs: #2462

Currently it have only two arguments, but that's not that useful, since procedural illustrations benefits from having a lot of parameters that can be tweaked at any time, and any float may end up being parametrized.

Also, I think their names should be configurable too, because custom names are better than one-letter name A B C D E.

[Keavon]

We can remove the secondary input, "B", and input all wires into the primary input of the node once this is implemented. That'll help make the node's signature less weird and more consistent.