Compute the index of a 1D array of spherical harmonic coefficients corresponding to i, l, and m.
index = YilmIndexVector (i, l, m)
index : integer : Index of a 1D array of spherical harmonic coefficients corresponding to i, l, and m.
i : integer : 1 corresponds to the cosine coefficient Ylm = cilm[0,:,:], and 2 corresponds to the sine coefficient Yl,-m = cilm[1,:,:].
l : integer : The spherical harmonic degree.
m : integer : The angular order, which must be greater or equal to zero.
YilmIndexVector will calculate the index of a 1D vector of spherical harmonic coefficients corresponding to degree l, (positive) angular order m and i (1 = cosine, 2 = sine). The index is given by l**2+(i-1)*l+m.