Add option to symmetrize fields

[AlexanderSinn]

Add option to symmetrize the fields (charge and currents before the field solve).

fields.do_symmetrize = 0

x-y plot with an offset beam:

fields.do_symmetrize = 1

• 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 great, thanks for this PR! I tested it with

mpirun -np 4 $HIPACE_dev inputs hipace.file_prefix=diags_dev/ \
    'electron.density(x,y,z) = 2.e22' \
    'ion.density(x,y,z) = 2.e22' \
    &> output_dev.txt
mpirun -np 4 $HOME/hipace/build/bin/hipace inputs hipace.file_prefix=diags_new/ \
    'electron.density(x,y,z) = 2.e22' \
    'ion.density(x,y,z) = 2.e22' \
    &> output_new.txt
mpirun -np 4 $HOME/hipace/build/bin/hipace inputs \
    fields.do_symmetrize = 1 \
    hipace.file_prefix=diags_4fold/ \
    &> output_4fold.txt

and this input file, and it works perfectly, the beam is identical after 3 time steps. We should also implement the two-fold symmetry.

Here initialised only 1/4 or the plasma, and multiplied the density by 4. When the symmetry is satisfied, it gives the same results with 4x fewer plasma particles.

[SeverinDiederichs]

I can confirm, I also used the feature for the ionization-induced hosing and it worked flawlessly.