Modifications according to new SAREF core

Issue#15 Stop hijacking SAREF Core terms (saref-portal#48 (closed)) Issue#14 Extensions copy declarations and definitions from SAREF Core (saref-portal#41 (closed)) Issue#17 Rename the GeoSPARQL prefix to geo (related to saref-portal#51 (closed)) Issue#12 Harmonise the way of representing geospatial information (saref-portal#26 (closed)) Issue#11 use domains/ranges axioms for clear cases and local restrictions otherwise (saref-portal#23) Issue#16 W3C Time ontology (saref-portal#50 (closed)) Issue#19 Add SKOS (saref-portal#67 (closed)) Issue#18 Comments in SAREF4CITY (saref-portal#56 (closed))