add the possibility to have a parabolic plasma profile

[SeverinDiederichs]

This PR adds a parabolic profile to the plasma.
The curvature is defined via plasma.parabolic_curvature.
The plasma profile is set to parabolic_curvature*r^2 + density,

Note, that very high density plasma particles at the boundary can cause problems and can cause the simulation to diverge.
Therefore, the curvature must be chosen with care.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[MaxThevenet]

Thanks for this PR! See comments.

[MaxThevenet]

Should we consider finalising this PR?

[MaxThevenet]

Did you check that this looks as expected? https://github.com/Hi-PACE/hipace/blob/f272837b1096bf09f55eb1e8e846233c748772b4/doc/source/run/parameters.rst

[SeverinDiederichs]

Did you check that this looks as expected? https://github.com/Hi-PACE/hipace/blob/f272837b1096bf09f55eb1e8e846233c748772b4/doc/source/run/parameters.rst

Now it does! Thanks

[MaxThevenet]

Looks great, thanks! See minor suggestion