HPC RSE: Matrix Acceleration and Unit Testing Framework

[Edward-RSE]

This PR will merge in the changes made for the HPC RSE project, relating to matrix parallelisation using CUDA and the implementation of a unit testing framework.

So in this PR, we have:

• The naïve implementation of using cuSolver to solve square* linear systems and calculate the inverse of square matrices. There are two new wrapper functions for this: solve_matrix and invert_matrix.
• Conditional compilation between CUDA and GSL. To enable CUDA, use the switch --with-cuda with the configure script.
• A unit testing framework, using Cunity. This was part of the project to avoid any regression being introduced as things were shuffled around and re-implemented in cuSolver. So far there are two test suites: one to test the GSL and cuSolver matrix code, and another example suite testing some of the Compton scattering functions.
• Cookbook level documentation on: 1) How to write unit tests and use the unit tests in the Cunity framework. 2) Usage, compilation and implementation documentation for the cuSolver code.
• Changes to the GitHub action to run (and potentially fail the action**) the unit tests

Apologies for the messy commit history and large number of files in the commit (most of which are CUnity). I had intended to go back and squash a number of commits together, but some commits had multiple parents which stopped me from being able to clean as far back as I would have liked. It's still possible to squash this feature branch into dev instead, if that's preferred.

*Both the GSL and cuSolver implementations are set up for square matrices. It should, however, be easy to extend both implementations to arbitrary shaped matrices. **That is fail when a unit test fails.

[jhmatthews]

I guess we can probably merge this @Edward-RSE ? Are the tests in the github workflow? I couldn't fully tell from the yml file but couldn't see the test output in the log.

[Edward-RSE]

I'm happy for this to be merged.

The unit tests run in the "build" part of the workflow, at the bottom of the "make gh_workflow_install" task. I think this link may take you to the right line in the logs.

It will be easy enough to move it elsewhere in the workflow if that's preferable.