XIOS: Account for U and V fields

[joewallwork]

XIOS: Account for U and V fields

Fixes #1066
Merges into #1063

Task List

• Linked an issue above that captures the requirements of this PR
• Defined the tests that specify a complete and functioning change
• Implemented the source code change that satisfies the tests
• Commented all code so that it can be understood without additional context
• No new warnings are generated or they are mentioned below
• The documentation has been updated (or an issue has been created to do so)
• Relevant labels (e.g., enhancement, bug) have been applied to this PR
• This change conforms to the conventions described in the README

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Change Description

The u and v fields are not properly handled in #1063 because they use special field types ModelArray::Type::U and ModelArray::Type::V, which aren't used by any other fields. Currently, these are duplicates of ``ModelArray::Type::H`, although this might not always be the case.

In this PR, these field types are handled in the XIOS I/O implementation. An assumption is made that ModelArray::Type::U and ModelArray::Type::V are consistent with ModelArray::Type::H, but NOTEs are included to indicate where this is relevant.

I also made use of using Type == ModelArray::Type; to make things a bit easier to read.

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Test Description

No changes to the tests were required.

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Documentation Impact

The XIOS documentation page has been updated to mention UField and VField. I also noticed that the domain names needed updating in the same section.