enable explicit solver with SI units

[SeverinDiederichs]

This PR enables the explicit solver with SI units.

In the explicit solver routine, all quantities are converted to normalized units, the field is solved in normalized units and then it is backtransformed to SI units (if run in SI units).

This can be tested with a script by @MaxThevenet which can be used in the blowout_wake example:

#! /usr/bin/env bash

HIPACE_EXECUTABLE=../../build/bin/hipace

rm -rf si_data
rm -rf normalized_data
# Run the simulation
mpiexec -n 2 $HIPACE_EXECUTABLE inputs_SI \
        hipace.file_prefix=si_data/ \
        hipace.bxby_solver=explicit \
        max_step=1

mpiexec -n 2 $HIPACE_EXECUTABLE inputs_normalized \
        hipace.file_prefix=normalized_data/ \
        hipace.bxby_solver=explicit \
        max_step=1

# Compare the result with theory
./analysis.py \
    --do-plot \
    --normalized-data normalized_data/ \
    --si-data si_data

To do so, all fixed weight calls must be removed from the analysis script.

By doing a more challenging test (increasing the beam density by a factor of 10, giving the beam an initial transverse momentum), normalized units and SI units agree to machine precision.

• 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]

could you resolve conflicts?

[MaxThevenet]

Looks great, thanks for fixing this!