VTK fails to open XDMF for large topologies

Currently the generated xdmf file stores the topology directly (introduced to support mixed topologies, see #31 (closed)). This section of the file can be very large, and for very large topologies (tested with 8 million elements) VTK fails to open the file.

The problem can be fixed by writing the additional required data to HDF5 instead.

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