Adder node: support + and - operations

[RemDelaporteMathurin]

Right now if we want to substract one input from another, we need to first multiply it by -1 with an amplifier then add them.

We could have something like this where all the nodes that are added are connected to the + sign and all the nodes substracted are connected to the minus.

[RemDelaporteMathurin]

Similarly to what we did in #89 we should probably overload a new Adder class to facilitate this