[CI] Add a workflow for the code coverage

[amontoison]

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[codecov-commenter]

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[amontoison]

@jfowkes @nimgould
I found how to do code coverage with Meson for Fortran code and upload the results to Codecov:
https://app.codecov.io/gh/ralna/GALAHAD

I added a badge to the README so that we can easily track its evolution.
Nick, the new workflow uses all our Fortran/C tests to determine which lines in the source code are covered or not.
Lines not covered are at greater risk of bugs (functions not tested or branches of the code that we never execute).

@AndrewLister-STFC
It might interest you to know that we can achieve this with Meson and gcovr for potentially the future "SparseHSL".

[nimgould]

Thanks, Alexis

Of course I appreciate that the GALAHAD tests are not as thorough as the HSL ones. Partially this is a lack of person power to do this justice, and part of it is because many GALAHAD packages are solving nonlinear problems for which there are inevitably pieces of code that are added as precautions without knowing if they will, indeed can ever, be executed. I honestly don't have the energy to improve this, I want to drive GALAHAD into new areas with the time I have left, not to worry about every small detail of existing codes that a 30 year old would, but a late 60 year old probably won't.