Use neXtSIM-DG date/duration formats in `Xios`

[jwallwork23]

Use neXtSIM-DG date/duration formats in Xios

Fixes #581

Fixes #582

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

Makes all use of the cxios_date and cxios_duration types private, including the conversion methods. Instead, the user interacts with XIOS calendar and duration operations through neXtSIM-DG's TimePoint and Duration classes.

Some TODOs are also addressed in xios_c_interface.hpp.

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Test Description

Xios unit tests are updated accordingly. They are now significantly less verbose.

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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]

One small comment on the capitalization of nextSIM.

Thanks @timspainNERSC. Will merge once the tests pass.