Add `unitaryPower` flag for QAM power selection

[daniel-s-w]

Hey there, as I've recently switched from MATLAB to Julia, I've discovered this nice package for digital communications simulation. As I've tried to port some code from MATLAB, I've found that there is no way to let the QAM generator output non-normalized values in the julia package. Thus, I've added this feature with backwards compatibility: To be able to switch the normalization of the output power for the QAM mapping, a flag has been added. Now one can decide between unitary output values and default QAM values (-1,+1,-3,+3, ...)

Daniel