GML temporal support

This pull requests brings GML temporal types and temporal predicates into a working state:

* Readers/writers for GML TimeObjects (gml:TimePeriod and gml:TimeInstant)
* Treat TimeSlice and TimeObject structures as GML objects, not as generic XML structures
* XPath evaluation on TimeSlices and TimeObjects
* Activated parsing of Filter 2.0.0 temporal operators
* In-memory evaluation of AnyInteracts temporal operator