type is specified. Previously, for mixed topology types, this would return
0.0. This would result in unreadable xdmf files being written to disk.

For mixed topology types, the function now examines the connectivity table
and parses topology type ids to count the number of elements. An improved
comment was added to XdmfTopology to explain how mixed topologies are stored.
A test for mixed topologies was also added, dealing with a quadrilateral and
a polyline in 2D.