XDMF support for isoparametric elements

[Chris Richardson]

Add support initially for elements with a quadratic geometry (i.e. geometry can be described by a P2 Vector function).

[Chris Richardson]

I've got a prototype working in XDMFFile::write_quadratic(u_geom, u_val) which writes a P2 Function on a geometry defined on a VectorFunctionSpace(mesh, "CG", 2).

[Chris Richardson]

master branch now supports XDMF quadratic geometry for output

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)