Add SSSOM or alignment-xml output

[cmungall]

I would like to analyze results using sssom tools; see:

• https://github.com/mapping-commons/sssom-py/issues/377

sssom-py has an adapter for the OAEI alignment-xml but this doesn't seem to be one of the formats produced?

It would be great if this tool could produce either format

[cmungall]

I have just been informed that the .rdf output is in fact alignment-xml