SPARQL access from XSLT / XPath

[redhog]

The grit.xml files only contain forward tripples starting with the current document, and xpath queries can become quite convoluted. As an alternative, a direct SPARQL query to the tripple store using an XPath extension function would allow a much more readable syntax while at the same time allowing access to all tripples, not just ones reachable by only forward tripples from the current document.