IO: Parallel Particle ASCII Output

[ax3l]

Write one file per MPI rank (and pot. OpenMP thread).
Update post-processing logic to process this.

Needed for #103

To Do:

• add a parallel test

X-ref:

• depends on Parallelize particle initialization #73
• Parallelize particle initialization #73 (comment)

[ax3l]

I also added a first MPI-parallel test (#103) now:

3: Test command: /home/axel/src/spack/var/spack/environments/warpx-omp-dev/.spack-env/view/bin/mpiexec "-n" "2" "/home/axel/src/impactx/build/bin/impactx.MPI.OMP.DP" "/home/axel/src/impactx/examples/fodo/input_fodo.in"
3: Environment variables: 
3:  OMP_NUM_THREADS=1
3: Test timeout computed to be: 1500
3: MPI initialized with 2 MPI processes
3: MPI initialized with thread support level 0
3: OMP initialized with 1 OMP threads
3: AMReX (22.04-9-g84a1f7d0715b) initialized
3: boxArray(0) (BoxArray maxbox(2)
3:        m_ref->m_hash_sig(0)
3:        ((0,0,0) (7,7,7) (0,0,0)) ((8,0,0) (15,7,7) (0,0,0)) )
3: 
3: Beam kinetic energy (MeV): 2000
3: Bunch charge (C): 0
3: Particle type: electron
3: Number of particles: 10000
3: Beam distribution type: waterbag
3: Static units
3: Initialized beam distribution parameters
3: # of particles: 10000
3: Initialized element list
3:  ++++ Starting step=0

I think we need to remove some hacks in main.cpp and make sure it uses multiple boxes (currently: 2?), in a follow-up.