Computes a list of points from a list of intervals and a initial point (<start>)
DX->X can be used for instance to get absolute pitches from intervals or to get onsets from durations:
Ex. (dx->x 6000 '(700 200)) => (6000 6700 6900)
Ex. (dx->x 0 '(1000 200 800 3000)) => (0 1000 1200 2000 5000))