Fix for invalid XPointer in XDMF output if `XdmfDomain::accept()` is called more than once...

If 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 XdmfExampleWrite.py:

        primaryDomain.accept(exampleWriter)

is replaced with two calls to accept():

        primaryDomain.accept(exampleWriter)
        primaryDomain.accept(exampleWriter)

then the resulting .xmf file is corrupted and contains invalid XPointers.

(See crayzeewulf/xdmf#2)

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.