PartSysBase and SimpleSOL+particles tweaks

[oparry-ukaea]

Description

Move handling of particle output frequency into particle system base class.

Type of change

• Refactoring/cleanup
  - [ ] Bug fix (non-breaking change)
  - [ ] New feature (non-breaking change which adds functionality)
  - [ ] API change (breaks downstream workflows and requires major semver bump)
  - [ ] Requires documentation updates

Testing

No new tests. Passes existing unit and integration tests.

Test Configuration:

OS: Ubuntu 22.04
Compiler: GCC 11.3.0 / OneAPI v2022.1.0
SYCL implementation: Hipsycl v0.9.2 / DPC++ v2022.1.0
MPI details: MPICH v4.0.2
Hardware: CPU (Intel Alder Lake)

Checklist:

• New and existing unit tests pass locally with my changes
• I have used understandable variable names
• I have run clang-format against my *.hpp and *.cpp changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
  - [ ] I have added tests that prove my fix is effective or that my feature works
  - [ ] Any new dependencies are automatically built for users via cmake
  - [ ] I have run cmake-format against my changes to CMakeLists.txt
  - [ ] I have run black against changes to *.py
  - [ ] I have made corresponding changes to the documentation

[oparry-ukaea]

N.B. I've only changed the parameter name in the SimpleSOL config file for now, since that's the only solver using PartSysBase at the moment.
I'll update the HW/LAPD solvers and their test/example configs on feature/3DHW.

[oparry-ukaea]

Last commit just adds some explanation of how 'num_particles_total' is used in SimpleSOL.

[will-saunders-ukaea]

Unit+Integration tests passed with hipsycl.