Implement Lie-group integrators for density propagation

[stestoll]

See Rasulov et al, Simulation and design of shaped pulses beyond the piecewise-constant approximation, J. Magn. Reson. 353, 107478 (2023) https://doi.org/10.1016/j.jmr.2023.107478 (benefits shown in Figs. 2 and 3)

See Spinach functions isergen and iserstep