fix mesh.gmsh for meshio >= 5.1.1
fix mesh.gmsh for meshio >= 5.1.1

In meshio PR #1234 the first CellBlock constructor argument was changed from
type to cell_type, breaking mesh.parsegmsh, which uses the object for older
gmsh formats. This patch restores functionality in a backwards compatible way
by switching from keyword to positional arguments.

wijnandhoitinga/nutils