Wrap XIOS API calls part 1: getters

Fixes #553

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
[ ] Updated the README or other documentation
[ ] Completed the pre-Request checklist below

Change Description

Currently, the XIOS functionality relies on axes, grids, fields, and files being defined in the user-defined iodef.xml file. This PR wraps the corresponding parts of the XIOS API so these can be read by neXtSIM-DG.

[x] Address methods in Xios.hpp not yet implemented.
[x] Interface XIOS functions from icfile.cpp (for handling files).
[x] Wrap axis getters from XIOS API.
[x] Wrap domain getters from XIOS API.
[x] Wrap grid getters from XIOS API.
[x] Wrap field getters from XIOS API.
[x] Wrap file getters from XIOS API.

Test Description

Update TestXiosInit_MPI2.cpp...

[x] To use the updated Xios.getCurrentDate implementation.
[x] To test axis getters.
[x] To test domain getters.
[x] To test grid getters.
[x] To test field getters.
[x] To test file getters.
[x] Fix write to file.

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
[ ] 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
[ ] File dates have been updated to reflect modification date
[ ] This change conforms to the conventions described in the README

nextsimhub / nextsimdg

Wrap XIOS API calls part 1: getters #555