Add example showcasing the use of `pregrad` and `postgrad`

[chaithyagr]

In #269 we added new features into mri-nufft: pregrad and postgrad.
We need examples showcasing its use through gradient plots and possibly trajectory curves.
We could potentially use different colours to showcase prephasor and rewinder

[paquiteau]

There is also the documentation of prewind/rewind and to differentiate with the recent work of #277

[chaithyagr]

I beliieve eventually we can get rid of prewind and rewind or make it use the trajectory state changer, as that allows for scanner compliant additions.. @Daval-G opinion?
For now, we wont touch those codes yet..