XdmfDomain::accept() is called more than once on the same
XdmfDomain object then the XPointers in the resulting
.xmf file are invalid.
For example, if the last line of the example file
is replaced with two calls to
then the resulting
.xmf file is corrupted and contains invalid XPointers.
After this fix multiple calls to
XdmfDomain::accept() produce the same
XPointers in the resulting
.xmf file. Need to run other unit tests to
make sure this does not break anything else.