Use `const` qualifiers in XIOS interface

[jwallwork23]

Use const qualifiers in XIOS interface

Fixes #583

Task List

• [x] Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
• [x] Implemented the source code change that satisfies the tests
• [x] Documented the feature by providing worked example
• [x] Updated the README or other documentation
• [x] Completed the pre-Request checklist below

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Change Description

I realised the Xios class contained many methods where read-only arguments aren't given the const qualifier. This is rectified in this PR.

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Test Description

Same goes for the XIOS tests.

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Pre-Request Checklist

• [x] The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
• [x] No new warnings are generated
• [x] The documentation has been updated (or an issue has been created to track the corresponding change)
• [x] Methods and Tests are commented such that they can be understood without having to obtain additional context
• [x] This PR/Issue is labelled as a bug/feature/enhancement/breaking change
• [x] File dates have been updated to reflect modification date
• [x] This change conforms to the conventions described in the README

[jwallwork23]

Looks good as far as it goes. But can any more of the member functions be declared const?

Thanks!

I don't think so? There are vectors that only get read in but I couldn't make them const because their data method gets called.