`mgf` does not take `Complex` numbers

[theogf]

The moment generating function is technically homomorphic in the complex space therefore restricting the input to be real does not make sense. Would a PR doing t::Real -> t::Number bet accepted?

[devmotion]

Sure. I guess the same could be done for (some) characteristic functions as well (https://en.wikipedia.org/wiki/Characteristic_function_(probability_theory)#Entire_characteristic_functions).