Creation of a pulse shaping filter
Over-Sampling Factor (number of samples by symbol)
Number of taps of the FIR filter approximation
Roll-off factor (for RC / SRRC filters)
Bandwidth-time product (for gaussian filter)
// Gaussian filter, 4 samples/symbol, 16 taps FIR, B.T. = 0.5 psf = psfilter_init('g', 4, 16, 0.5);