[datafeed] add parser to import cubicweb xml this parser is configurable through a mapping, which basically tells which entity's relation should be copied. Multiple actions to control backport of data are available: * 'copy': copy the external entities locally (though marked as coming from the data feed source) * 'link': find similar entity internaly (e.g. for states) * 'link-or-create': try to find a similar entity internaly, and create it if necessary (won't be marked as coming from the data feed source, e.g. for tags)