Trajectory display overhaul

[Daval-G]

This PR consists of the following features:

• New display functions to show 2D and 3D gradients with several options.
• Updates on the trajectory display functions to properly account for acquisition parameters and hardware constraints.
• Fixes on wrong units in comments and documentation.
• A new set of util functions to manipulate and convert trajectory/gradients/slew rates and check gradient constraints, as previous functions were kind of twisted to serve multiple purposes at once, notably for IO.

Currently, the documentation still needs to be written and examples updated as they are the first ones impacted by the display changes. Here is a simple rendering:

[philouc]

We should probably set up a discussion in the coming weeks to see notably how to interface your nice contributions to mri-nufft with the brand new fMRI simulator developed by @paquiteau

[paquiteau]

Seems very promising, the gradient plot are exactly what I had in mind. There is surely some docs effort to make but this is really good.

[paquiteau]

It keeps getting better !
The displayConfig can be simplified though, see my suggestions

[paquiteau]

LGTM !