3D radial trajectory bundle (clean)

[Daval-G]

This PR is a cleaned version of PR #113 and adds four 3D radial trajectories from the literature mentioned in issue #106:

1. initialize_3D_phyllotaxis_radial from "Spiral Phyllotaxis: The Natural Way to Construct a 3D Radial Trajectory in MRI" Davide Piccini et al. (2011). Radial shots are oriented following a Fibonacci sphere lattice.
2. initialize_3D_golden_means_radial from "Temporal Stability of Adaptive 3D Radial MRI Using Multidimensional Golden Means", Rachel W. Chan (2009). Radial shots are oriented following Fibonacci-based non-repeating angles.
3. initialize_3D_wong_radial from "A Strategy for Sampling on a Sphere Applied to 3D Selective RF Pulse Design", Sam T. S. Wong and Mark S. Roos (1994). Radial shots are oriented following one or multiple spiral(s) over the k-space sphere.
4. initialize_3D_park_radial from "A radial sampling strategy for uniform k-space coverage with retrospective respiratory gating in 3D ultrashort-echo-time lung imaging", Jinil Park et al. (2016). This pattern is based on Wong et al. (1994) from initialize_3D_wong_radial, enforcing a single spiral and ordering shots in a different way.

Documentation & examples coming soon.
The illustration below shows the tips of each radial shot for each of the four patterns.

[Daval-G]

@chaithyagr to answer your suggestion from #113 (comment), AZTEK is currently listed in issue #106 but I hadn't realized it was radial. I will comment later to state whether I include it in this PR or in another one.

[Daval-G]

@chaithyagr This point (#113 (comment)) has been raised a few times now, can you create an issue ? Or do we see it done extra short term ?

[chaithyagr]

@chaithyagr This point (#113 (comment)) has been raised a few times now, can you create an issue ? Or do we see it done extra short term ?

I have added it as an additional task in #77 .

[paquiteau]

Apart from the docstring and examples this is good for me :)