Add Isobuster's DFXML output to METS file

[bitsgalore]

BUT note that Isobuster DFXML does not validate against the official XSD https://github.com/KBNLresearch/iromlab/issues/59#issuecomment-356345043

ALSO Isobuster's XML sometimes contains illegal characters:

https://gist.github.com/bitsgalore/ee8a6b7fc519ef282d3a85df57cf5182

Line 95:

https://gist.github.com/bitsgalore/ee8a6b7fc519ef282d3a85df57cf5182#file-isobuster-report-xml-L95

Line 134:

https://gist.github.com/bitsgalore/ee8a6b7fc519ef282d3a85df57cf5182#file-isobuster-report-xml-L134

Report this to Isobuster!

[bitsgalore]

Isobuster 4.2's %DFXML variable fixes the above issue; reserved characters are now replaced by entity references). See also (Iromlab):

https://github.com/KBNLresearch/iromlab/issues/59

[bitsgalore]

Done: https://github.com/KBNLresearch/omSipCreator/commit/0b46be24d6c6b8a1fadb23547db64c9b38cf05d9

TODO: put in separate dfxml namespace to avoid ns conflicts

[bitsgalore]

Added namespace: https://github.com/KBNLresearch/omSipCreator/commit/7986ea133c04a80cd0d277675b4ccefa00e5035e