XIOS duration handling

[jwallwork23]

XIOS duration handling

Closes #628 Closes #636

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

This PR addresses a few minor concerns:

• XIOS interface methods related to output frequency and split frequency now work with nextSIM-DG's Duration objects, rather than accepting XIOS' duration strings, which differ in confusing ways.
• Implement private methods for converting between XIOS and nextSIM-DG's time duration formats.
• Implement a private method for converting between C and C++ string formats, since this is done repeatedly in the XIOS interface.
• Disallow "PY-M-DTH:M:S"-style input to Duration. Namely, do not allow months to be specified because they have arbitrary length and the convention does not align with XIOS, which could cause confusion.

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Test Description

Unit tests are updated accordingly. I also thought it would be a good idea to test more rigorously that durations with different units are treated appropriately.

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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