Simplify the pathinfo interface

[luisfabib]

In dipolarkernel the dipolar pathways are specified as a list of amplitude, refocusing time, and harmonic parameters:

pathinfo = [[],[],[]]
pathinfo[0] = [lam0,   nan, nan]    # unmodulated part, gives offset
pathinfo[1] = [lama,   0,   1]      # main modulation, refocusing at time zero
pathinfo[2] = [lam21, tau2, 1]      # 2+1 modulation, refocusing at time tau2

for the unmodulated pathway the refocusing time and harmonic must be specified as nan-values. This is a leftover from the MATLAB interface. Since the Python interface is list-based, the unmodulated pathway could be specified as a list with just the amplitude:

pathinfo = [[],[],[]]
pathinfo[0] = [lam0]             # unmodulated part, gives offset
pathinfo[1] = [lama,   0,   1]   # main modulation, refocusing at time zero
pathinfo[2] = [lam21, tau2, 1]   # 2+1 modulation, refocusing at time tau2