Make conjugateComlex and sign constexpr

This MR marks two simple functions from math.hh as constexpr. The type std::complex provides since c++20 also constexpr members.

requested review from @santiago.ospina

assigned to @simon.praetorius

I think that this is fine. I just wanted to note that std::signbit is probably a better alternative since it will be able to handle standard SIMD types, but it's only constexpr since C++23.

approved this merge request

merged

mentioned in commit d46fc4f1