Option to symmetrize plasma

[SeverinDiederichs]

Update of #891 (there was an index missing)

This PR proposes to symmetrize the plasma at initialization. This prevents the numerical seeding of the hosing instability for a finite temperature.

The symmetrization increases the level of noise due to the temperature in comparison to using the same amount of random particles (symmetrization increases the number of particles by 4). As shown below. As one can see, the noise is quite symmetric in x (see e.g., at the axis at -1 or -2.). Note that one could also symmetrize the plasma in z.

The symmetrization from this PR is sufficient to prevent the hosing instability to be seeded from the numerical noise of the temperature:

• 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

[AlexanderSinn]

LGTM