Prepare for setting XIOS up via nextSIM-DG

[jwallwork23]

Prepare for setting XIOS up via nextSIM-DG

Partially addresses #633.

This PR reworks the XIOS setup workflow in preparation for farming much of it off to happen automatically via nextSIM-DG. For example, when ModelArray::setDimension is called with ModelArray::Dimension::X, we can have the XIOS handler set the local and global X sizes correspondingly.

Main changes:

• Use xy_domain and z_axis consistently in XIOS read and write demos.
• Reorder operations in XIOS read and write demos, with TODOs planning out how the setup steps will be automated via nextSIM-DG.
• Automatically create an XIOS Grid grid_2D upon creating an XIOS Domain with the special name xy_domain.
• Automatically create an XIOS Grid grid_3D upon creating XIOS Domain and Axis with the special names xy_domain and z_axis, respectively.

Other minor changes:

• Finish moving from initialising strings with {} to =.
• Add a check to ensure local and global sizes match for Z dimension because we only MPI parallelise in the horizontal.

[jwallwork23]

Thanks for the review, Tim!