This method is used to instantiate a GML Abstract CRS
addAxis(axis)
Adds an axis, object of class GMLCoordinateSystemAxis
delAxis(axis)
Deletes an axis, object of class GMLCoordinateSystemAxis
References
ISO 19136:2007 Geographic Information -- Geographic Markup Language.
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=32554