Allowing `Mesh(filename)` to work with `.xdmf` format

[Bin LI]

It would be convenient to make Mesh(filename) work with .xdmf format. The following lines need to be modified if the file is .xdmf format.

Mesh::Mesh(MPI_Comm comm, std::string filename)
  : Variable("mesh", "DOLFIN mesh"), Hierarchical<Mesh>(*this), _ordered(false),
  _mpi_comm(comm)
{
  File file(_mpi_comm, filename);
  file >> *this;
}

[Prof Garth Wells]

Use the mailing list for proposals. There are reasons why we wish to go via a file object for IO.

Moreover, the proposed fix will not work. The File interface no longer supports XDMF.

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)