Mb/charge exchange kernel new

[mbukaea]

Description

Please include a summary of changes, motivation and context for this PR.

Fixes #178
Fixes #194

Type of change

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

Testing

Please describe the tests that you ran to verify your changes and provide instructions for reproducibility. Please also list any relevant details for your test configuration.

• Test Foo in /test/path/to/file_for_test_Foo.cpp
• Description of test Foo
• Test Bar in /test/path/to/file_for_test_Bar.cpp
• Description of test Bar

Test Configuration:

• OS:
• SYCL implementation:
• MPI details:
• Hardware:

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

[will-saunders-ukaea]

Guessing these should be calls to get_initial_* and the calls to get_initial_* should in turn call compute_initial_* and/or call the compute methods in some setup/constructor.

[will-saunders-ukaea]

I wouldn't store momentum when we have mass and velocity already stored on the particles. Unless we need momentum in enough places to amortise the cost of the extra data movement.

[will-saunders-ukaea]

I think you should generalise this class for n dimensions (where the number of dimensions are passed in the constructor). Otherwise we will end up with 2,3 copies of the same function with the only difference being an index.

[will-saunders-ukaea]

These diffs look like a find/replace edited an additional file?