Add AZTEK trajectory

[DlWAT]

Addition of a function to create AZTEK trajectories

[Daval-G]

@DlWAT Thank you for this contribution
Could you edit the PR title to make it more explicit, and could you provide a more detailed PR description ?
The code style seems to be old school, is it taken from somewhere and/or directly adapted from another language ?
I will try to provide a review this week, I expect that the code can be considerably reduced using native numpy functions.

[DlWAT]

The code was originally in C, but I translated it. I had a more python-friendly version, but I was less sure of the result.
The code originates from this github. But it only gives the end of the spokes, i.e. the max gradient (here 32768). I've added an equal distribution of points on each spoke starting from the center, and put the point coordinates between [-0.5 ; 0.5].
I can't really explain the three parameters speed, shuffle and twist, as I haven't worked on them.
https://github.com/BioMaps-MRI/AZTEK
You can find more explanation in this article : AZTEK: Adaptive zero TE k-space trajectories.

[Daval-G]

Thank you for those informations. AZTEK is a trajectory we have been wanting to add for quite some time now (listed in issue #106).
Let me know to which extent you would like to contribute to its integration. For example, we usually add examples to the gallery for each new trajectory, but I can take care of that. Similarly, do you wish to make changes yourself during the review process as assignee or should we make the changes and validate them with you ? (to ensure fidelity to the paper, as from my understanding you are involved with the author team)