Improve XDMF interface

[Chris Richardson]

The current XDMF interface has some inconsistencies and could do with a clean-up. I propose the following objectives:

1. It should be made clear when a new file is created, and when an existing one is appended to.
2. Time series should never be created automatically, but only when explicitly asked for.
3. There should be a clear interface for writing a Mesh with volume markers, and facet markers, and the existing automatic MeshDomain support should be dropped.
4. Possibly support XDMFFile::read(Function&) for CG1, with a warning. Future support for higher order elements may be coming.

[Chris Richardson]

Also would be good to have Xdmf3 compatibility.

[Chris Richardson]

Most of these points are now resolved, new features will be added on 2017.1