add(::Real, ::Wirtinger) throws StackOverflowError

[simeonschaub]

When trying to add a Wirtinger derivative to either Zero(), One(), a real number or DNE(), currently a StackOverflowError is thrown. It would probably make sense for real numbers to be handled as Wirtinger(x, Zero()), although one would still have to decide how to handle DNE().

[simeonschaub]

The problem here seems to be, that the constructor for Wirtinger falls back to just adding primal and conjugate if both are real, so the implementation for add_wirtinger fails to change any types and gets stuck in an infinite loop. To have this fall-through behavior in the constructor seems problematic in the first place – I'm not even sure this behavior is actually valid in all cases. If this behavior is valid, one should probably have an extra method, that behaves like the constructor does now and have the constructor always return a Wirtinger type.