Add new MRI pulse sequence: Stimulated-echo (STE) pulse sequence

[marovst]

Use as resource. We might find a definition here:
https://mriquestions.com/stimulated-echoes.html

[lzehl]

@tgbugs for definition

[tgbugs]

A stimulated echo pulse sequence is a mri pulse sequence that uses 3 pulses where the 2nd and 3rd pulse interact in the object imaged to produce a stimulated echo.

unfortunately circular because we need to explain what a stimulated echo is

here is @lzehl 's draft definition (which doesn't fit our usual pattern)

Stimulated echo: An echo produced by a three-RF-pulse sequence, typically three 90° pulses, in which magnetization is stored longitudinally after the second pulse and later returned to the transverse plane by the third pulse.

Stimulated echo: An echo produced by a three-or-more-RF-pulse sequence in which transverse magnetization is stored temporarily as longitudinal magnetization by one pulse and later returned to the transverse plane to rephase by a subsequent pulse.