Test: Charge Deposition

[ax3l]

Test the charge deposition logic & scaling of values.

• fix charge (individual particle weighting) of MPI-parallel bunch init
• add summing to MPI-enabled tests and generalize to work without duplicate cells as sum() does by default

Ran via

cmake -S . -B build -DImpactX_PYTHON=ON
cmake --build build -j 2
cmake --build build -j 2 --target pip_install

# serial
python3 -m pytest -v -ssss tests/python/test_charge_deposition.py
# parallel
mpirun -n 4 python3 -m pytest -v -ssss tests/python/test_charge_deposition.py

Prepares #162
Depends on:

• Fix: MultiFab Iterator (first) AMReX-Codes/pyamrex#50
• RealBox: Extra Constructor AMReX-Codes/pyamrex#61
• Add: MultiFab::sum_unique AMReX-Codes/pyamrex#63