XIOS implementation of writeDiagnosticTime

Fixes #917
Merges into #928
Will be followed by addressing #929

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

Change Description

This PR provides an XIOS implementation of writeDiagnosticTime. It completes the set of member functions needed in ParaGridIO_Xios, so the other copied functions are deleted and ifdef-ed out in the header.

Much like in #928, this required a new config section XiosDiagnostic (c.f. XiosForcing).

During my work on this, I noticed #933 and #935.

Test Description

$u$-velocity field written as a diagnostic in the XIOS write test, much like how it is read in the forcing test added in #928.

Changes to XIOS read test for consistency.

Documentation Impact

Diagnostics are mentioned in the XIOS section of the docs.

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
• This change conforms to the conventions described in the README