transposition of meshio cell_data keys

[gdmcbain]

As mentioned in the context of 0.4.0 release #299, nschloe/meshio#626 has transposed the keys of meshio.Mesh.cell_data to name before cell-type. This is in meshio master but hasn't been released yet.

Using meshio master, this breaks ex21.py which raises

/home/geordie/src/scikit-fem/skfem/importers/meshio.py:114: UserWarning: Unable to load tagged boundaries/subdomains.
  warnings.warn("Unable to load tagged boundaries/subdomains.")
Traceback (most recent call last):
  File "docs/examples/ex21.py", line 28, in <module>
    dofs['fixed'].nodal['u^1'],
TypeError: tuple indices must be integers or slices, not str

The problem is that the transposition needs to be reflected in

scikit-fem/skfem/importers/meshio.py

Lines 77 to 78 in f8909f5

	if meshio_type in m.cell_data and 'gmsh:physical' in m.cell_data[meshio_type]:
	elements_tag = m.cell_data[meshio_type]['gmsh:physical']

and elsewhere following.

[gdmcbain]

Subsumed by #321.