Regression tests for solver examples

[oparry-ukaea]

Description

Sets up a regressionTests gtest exec. A test is run for (almost) every examples/[solver_name]/[example_name] subdirectory.

Main additions are

• a SolverRegTest gtest fixture (sub-classed for each solver)
• a Python script to generate HDF5 regression data (reads example output and uses NekPy to generate "equispaced" points for each field.
• Regression data at test/regression/examples/[solver_name]/[eg_name].regression_data.h5

Particle output ignored for now (so no test for Electrostatic2D3V/TwoStream yet).

Resolves #275.

Type of change

• New feature (non-breaking change which adds functionality)
• Requires documentation updates

Testing

Existing unit, integration tests pass; new regression tests added.

Test Configuration:

• OS: Ubuntu 22.04
• Compiler: GCC 11.3.0 / OneAPI 2024.1.0
• SYCL implementation: AdaptiveCpp 24.06.0 / DPC++ 2022.1.0
• MPI details: MPICH 4.0.2 / Intel MPI 2021.8 Build 20221129
• Hardware: CPU (Intel Raptor Lake)

Checklist:

- [ ] I have added tests that prove my fix is effective or that my feature works

• New and existing unit tests pass locally with my changes
• Any new dependencies are automatically built for users via cmake
• I have used understandable variable names
• I have run clang-format against my *.hpp and *.cpp changes
• 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
• 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

[oparry-ukaea]

Tests for SimpleSOL and Diffusion solver examples are working.
H3LAPD test work for 2DRogersRicci, but not for HW examples yet.