modulateSignal provides a sinusoidal signal modulated by a generic piecewise or continuous
envelope function assigned by the user. The frequency and the phase on the signal can also be assigned.
The main advantage is to create a signal inside a general envelope function without using any toolbox.
Furthermore, such a function can be seen as the opposite of the envelope function.
Exponential, harmonic and polynomial functions are supported to create envelope functions.
The signal produced will have zero mean. See the help for a more detailed description.