Computes the effect of a gradient pulse on the sample average density matrix using Edwards formalism. It is assumed that the effect of diffusion is negligible, that the gradient is linear, and that it is antisymmetric about the middle of the sample. Syntax:

      rho=grad_pulse(spin_system,rho,g_amp,s_len,g_dur,s_fac)


Arguments:

             rho   - spin system state vector

               L   - system Liouvillian

           g_amp   - gradient amplitude, Gauss/cm

           s_len   - sample length, cm

           g_dur   - gradient pulse duration, seconds

           s_fac   - gradient shape factor, use 1 for