Set up SonarCloud integration

[cmacmackin]

On GitLab, the CI made use of a private UKAEA instance of SonarQube. This will no longer be available, now that the code is hosted on GitHub. Instead it should use SonarCloud, the SaaS version of SonarQube. The project will need to be made open source before it can be registered with SonarCloud for free.

[P-S-S-M]

Everything should be good with the sonarcloud checks and the remaining issues but we need the UKAEA sonarcloud admin (@cmacmackin I think) to setup a New Code Definition.

[cmacmackin]

I've done that now (I think).

[P-S-S-M]

Yeah all working now, issue should be able to be closed