How to handle complex numbers?

[gdalle]

Level 1 (good)

Internal operations on Complex{Dual} and Complex{Tracer} should be supported, as long as there are Real numbers at the start and at the end.

Requires overloading conversions and promotions, not more.

Level 2 (still debatable)

We could decide how to initialize tracing with complex input, and read out the tracers to create a Jacobian.

[adrhill]

I think we can closes this for now, until the need for it arises.

If you're a user and need to compute sparsity patterns through computations involving complex numbers, please open an issue!