Add derivative shape factors

[AlexanderSinn]

Runtime in s of ExplicitDeposition() with 2 2 ppc of electrons and ions with 1024*1024*2048 cells:

order	dev	PR centered	PR nodal	PR analytic
0	3.007	1.881	1.778	N/A
1	5.161	2.838	2.360	1.778
2	7.645	4.632	3.678	2.491
3	11.130	6.900	5.534	3.691

Depos order = 2, dev:


PR centered (Numerically identical, new default):


PR nodal:


PR analytic:

• 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 minor comments in the text. A few additional points:

1. Could you add a PR description and illustrate the effect of this PR? For instance, on GPU, could you show some xy or xz cut for nodal and centered depositions, and give the runtime difference?
2. I think a little convergence test would be very nice here, increasing transverse resolution by factors of 22, 44 etc. and show e.g. the beam emittance after 100 fixed time steps, with each depositions.

I would be happy to discuss this, and I believe point 2 above could show very nice results!

[MaxThevenet]

Great, thanks! Although not urgent, a convergence test could demonstrate that this new scheme is both faster and more accurate, which would be a very nice result!