URL and regex/XPath based facets

[nuest]

Instead of (or in addition to) XPaths for validating label availability, extend transformer files to support a URL and regex or URL and XPath based mechanism. This could be used for example to determine the last available observation from a Sensor Observation Service. The URL is a GetObservation request, the regex picks up (or an XPath) picks up the latest value.

These options make the transformation file more complex so #18 becomes more relevant.