4D round q-Gaussian generator

[elleanor-lamb]

Description

Adds 4D q-gaussian generator to create normalised coordinates in 4D.
Beam is round and q is the same in x and y.
Example in xpart/examples/particles_generation/008_generate_q_gaussian.py

Checklist

Mandatory:

• [x ] I have added tests to cover my changes
• [x ] All the tests are passing, including my new ones
• [x ] I described my changes in this PR description

Optional:

• [x ] The code I wrote follows good style practices (see PEP 8 and PEP 20).
• [ x] I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts

[elleanor-lamb]

Need to add tests and test in GPU context. Would also appreciate some feedback on the numerical stability of generate_radial_distribution inside /transverse_generators/q_gaussian_round.py