Internal soundgen function called by dPhase(). Propagates phase using the "vocoder done right" algorithm, as in Prusa & Holighaus 2017 "Phase vocoder done right".
phasePropagate(i, dp_hor, dp_ver, magn, phase_new, tol, bin_width, alpha)
analyzed frame
time and frequency partial derivatives of phase
matrix for storing the new phase
tolerance of "vocoder done right" algorithm
width of frequency bin, Hz
stretch factor