Grad amplitude bug in example sequence write_tse.seq

[h3lg3]

The rf refocussing pulse gradient for slice selection uses the amplitude from the excitation pulse gradient. I believe the amplitude from the previously generated gz gradient for the refocussing pulse should be used instead.

https://github.com/imr-framework/pypulseq/blob/0398b12bc1d86c1997ed5ad34242a22cc95003bd/examples/scripts/write_tse.py#L91

    gs_ref = pp.make_trapezoid(
        channel='z',
        system=system,
        amplitude=gs_ex.amplitude, # here gz.amplitude should be used
        flat_time=t_refwd,
        rise_time=dG,
    )