WIP: read multiple Grids in XDMF

[gdmcbain]

This is for #568.

[gdmcbain]

This includes #609.

[gdmcbain]

Thus far, this does read multiple Grids well enough to read the MWE; but perhaps this should go further and:

• Distinguish the cells read from different grids.
• Write multiple grids instead of TopologyType="Mixed".

The exact desired behaviour does depend on bit on the interpretation of tagged subsets of elements in meshio #290.

[gdmcbain]

Given the current code

meshio/meshio/xdmf/main.py

Line 251 in 23fa4ef

cells[xdmf_to_meshio_type[cell_type]] = data

plus the new code in #610 to read multiple Grids, subsequent grids with the same cell_type will clobber those preceding them. That can't be the desired behaviour. I suppose the new cells should be appended but should they be distinguished from those already read? How? Like Gmsh MSH2

cell_data[xdmf_to_meshio_type[cell_type]]["gmsh:physical"]

or something like MED for which I see

meshio/meshio/med/_med.py

Lines 105 to 114 in 23fa4ef

	cell_tags = {}
	if "ELEME" in fas:
	cell_tags = _read_families(fas["ELEME"])
	# Construct the mesh object
	mesh = Mesh(
	points, cells, point_data=point_data, cell_data=cell_data, field_data=field_data
	)
	mesh.point_tags = point_tags
	mesh.cell_tags = cell_tags

though I'm not familiar with the MED format so don't know what that attribute would usually be expected to contain.

…Ah, I've just noticed that meshio.Mesh has sprouted some new attributes! node_sets and element_sets. I assume that these should be renamed point_sets and cell_sets (as per #493). The cell_sets could be just what's wanted here (and for #290).

[gdmcbain]

This now works again, following the switch from lxml, correctly reading the MWE of #568 with its two Grids sharing Geometry. It also records the two Topologies to cells and cell_sets (the latter partially fulfilling #290).

It remains to also write multiple grids, thereby:

• avoiding TopologyType="Mixed"
• enabling testing via test.helpers.write_read

[nschloe]

The smaller a PR, the better; we can do multiple grids later. Best bring this one here into a reviewable state.

[gdmcbain]

Yes, right, sorry; I'll split this up.

[gdmcbain]

1. xml.etree.ElementTree usage #621

[gdmcbain]

1. xml.etree.ElementTree usage #621
2. read multiple XDMF Grids #622