cubicweb-postgis #4846374 Add point-coordinates extraction functions [validation pending]

Functions such as ST_X() can be used to extract a single coordinate from a POINT.

prioritynormal
typeenhancement
done in0.3.1
load0.500
load left0.000
closed by#d471ed6c78d5 Add point coordinates extraction functions (closes #4846374)
patchAdd point coordinates extraction functions (closes #4846374) [applied]