Correct ion charge deposition

[SeverinDiederichs]

This PR introduces a new way to deposit the ion charge. It resolves #122.
It depends on #155! This PR can only be merged after #155 is merged. If this one is merged first, it makes #155 redundant and it can be closed.

Previously, a uniform ion charge was assumed and simply added during the plasma current deposition.
Now, the number of slices was increased to 5, with the 5th slice being reserved for the ion charge density.

The current deposition was modified to be able to deposit the charge directly to the ion rho slice.

It was made more modular by adding boolean options which quantities to deposit (Jx + Jy, Jz, rho).
This has the advantage, that the parameter WhichSlice::Next, This, RhoIons now only affects in which slice something is deposited and nothing else (previously, it under the hood also managed what to deposit).

This PR showed a bug with the treatment of the boundaries.
Although, the currents should be exchanged with SumBoundary, it gives the wrong result (a residual current on the edges, leading to numerical noise). Therefore, in this PR FillBoundary is added for the exchange of the currents. This needs to be addressed in a separate issue.

The benchmarks had to be updated, because spurious current deposition from the previous implementation was updated. Comparison of the lower box of rho from the following input script (the y axis label should be x, not rho)
inputs_normalized.txt
Old:

With this PR:

I do realize that rho in the slice_deposition benchmark was changed by many orders of magnitude, but this is correct. I tested it for a linear plasma wake with slice deposition and host-device and it worked fine. I would like to point out that Bz decreases by many orders of magnitude and it should be 0 for symmetric problems.

• 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]

Looks good to me, I must say I didn't check all calls to DepositCurrent with the new bool arguments.