Emulates the effect of a gradient sandwich on the sample average density matrix using Edwards formalism. It is assumed that the effect of diffusion is negligible, that the gradients are linear, and that they are antisymmetric about the middle of the sample.

## Syntax

    rho=grad_sandw(spin_system,L,rho,P,g_amps,s_len,g_durs,s_facs)


## Arguments

             rho   - spin system state vector

L   - system Liouvillian

P   - total propagator for all events happening

g_amps   - row vector containing the amplitudes of

s_len   - sample length, cm

g_durs   - row vector containing the durations of

s_facs   - shape factors of the two gradients, use
[1 1] for square gradient pulses


## Outputs

     rho - spin system state vector, integrated over
the spatial coordinate


## Examples

See examples/fundamentals/gradient_test_2.m file for an example of using this function.

## Notes

1. WARNING This function integrates over the spatial coordinate after the gradient pulses are completed - subsequent gradient pulses would not refocus the magnetisation that this function removed because it is removed mathematically. If your experiment has multiple gradient pulses, you must model the spatially distributed spin dynamics explicitly using imaging.m context.
2. More information on the subject is available in Luke's paper (http://dx.doi.org/10.1016/j.jmr.2014.01.011).