XIOS: Account for separate restart and forcing files

[jwallwork23]

The draft implementation for reading forcings in #916 currently assumes that forcings are read from the restart file. It would be better if these files could differ.

[TomMelt]

Thought it would be helpful to add my comment here. In restart file I think we need to support DGField, CGVector and maybe VertexField as well.

nextsimdg/core/src/ParaGridIO_Xios.cpp

Lines 142 to 146 in 116506e

	// ASSUME all forcings are HFields: finite volume fields on the same
	// grid as ice thickness
	HField field(ModelArray::Type::H);
	field.resize();
	state.merge(ModelState { { { fieldId, field } }, {} });

[jwallwork23]

Thought it would be helpful to add my comment here. In restart file I think we need to support DGField, CGVector and maybe VertexField as well.

This will be done in #910 and #911.