OCSADV-27: Model and parser for VoTable

Model, Parser and tests for VoTable xml. The main public function takes an xml node and returns a list of resources containing sidereal targets. Likely, there will be changes to the model as it is integrated into the AGS and UI

As much as possible the parser tries to retain parsing errors, but at the same will extract as many valid targets as possible
I'm not too happy with this line. I'd like to pick the first error if any or let it go through if no errors are present
Any ideas?