XIOS: account for DGField and VertexField

[jwallwork23]

XIOS: account for DGField and VertexField

Fixes #831
Merges into #872
Towards #850

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Change Description

This PR overhauls the XIOS implementation to account for DGField and VertexField, as well as HField, which was already covered. This required setting up separate XIOS grids for each field type, themselves formed of different combinations of axes and domains.

• HGrid is the simplest case, with a x-y domain (HDomain).
• VertexGrid has its own xvertex-yvertex domain (VertexDomain) and also an axis with ncoords components (VertexAxis).
• DGGrid uses HDomain, along with an axis with dg_comp components (DGAxis).

The PR also fixes misc typos, inlines setFieldOperation with 'instant' mode (which is always assumed), and drops getFieldOperation as no longer needed.

Checklist

• Size mismatch for writing DGField - see debug_other_fields_write.mdb.
• Wrong ModelArray type - see debug_other_fields_read.mdb.
• Size mismatch for reading VertexField and DGField (possibly related to above).
• Cleanup

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Test Description

Various XIOS tests needed reworking to account for this change.

[jwallwork23]

Added and used ModelArray::Base in e583946. This is currently hooked up in discontinuousgalerkin but not finitevolume. What base types would make sense in the latter? Facet or just Vertex?

[timspainNERSC]

That all looks reasonable.

[jwallwork23]

I'll hold off merging this until #744 is merged to avoid slowing it down.

[jwallwork23]

Created branch issue831_xios-other-fields_halo-exchange in preparation for merging after #744. [skip ci]