Pull Request Title

Fixes #586

Fixes #570

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

Change / test Description

The XIOS tests are currently all in one file, which has got rather long. It would be better to modularise these into different tests for each XIOS concept. This is straightforward now that the iodef.xml file is minimal - the same one can be used for each test.

In addition, for many of the tests simpler objects could be used, e.g., axis {0, 1} rather than {0, 1, ..., 29}.

This PR also fixes a warning raised due to XIOS.

[x] Separate out calendar tests
[x] Separate out axis tests
[x] Separate out domain tests
[x] Separate out grid tests
[x] Separate out field tests
[x] Separate out file tests
[x] Separate out write tests
[x] Test gridAddAxis
[x] Test gridAddDomain
[x] Test fileAddField
[x] Test writing fields of different dimensions
[x] Fix INFO warning (issue #570)

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

nextsimhub / nextsimdg

Split up XIOS tests per concept #588