Clean up the SimpleSOL solver and structure it more like H3LAPD

[oparry-ukaea]

Description

Tidy up and simplify SimpleSOL, in preparation for adding a common solver base used by both SimpleSOL and H3LAPD.

Type of change

Please delete options that are not relevant.

• [x] Cleanup / refactoring

Testing

n/a - passes existing SimpleSOL 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:

• [x] New and existing unit tests pass locally with my changes
• [x] Any new dependencies are automatically built for users via cmake
• [x] I have used understandable variable names
• [x] I have run clang-format against my *.hpp and *.cpp changes
• [x] I have run cmake-format against my changes to CMakeLists.txt
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings