WIP: XIOS implementation of ParaGridIO

[jwallwork23]

XIOS implementation of ParaGridIO

Fixes #552

Task List

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

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Change Description

Reimplementation of the ParaGridIO class using XIOS for I/O. It will automatically be used if nextSIM-DG is compiled with XIOS enabled.

The read and write methods formerly in the Xios handler class are moved to the XIOS implementation of ParaGridIO. This is because ModelArray is going to get XIOS-related functionality and we need to avoid circular dependencies.

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Test Description

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Pre-Request Checklist

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

[jwallwork23]

Suggestion from @timspainNERSC: instead of moving the read and write methods from Xios to ParaGridIO_Xios, it might be sufficient to just forward-define the ModelArray class in Xios.