[st] add the ST_CENTROID function

cubicweb_postgis/site_cubicweb.py

@@ -338,6 +338,13 @@ class ST_MAKEENVELOPE(FunctionDescr):
     rtype = 'Geometry'
 
 
+class ST_CENTROID(FunctionDescr):
+    minargs = 1
+    maxargs = 1
+    supported_backends = ('postgres',)
+    rtype = 'Geometry'
+
+
 register_function(ST_EQUALS)
 register_function(ST_INTERSECTS)
 register_function(ST_INTERSECTION)
@@ -376,3 +383,4 @@ register_function(ST_M)
 register_function(ST_EXTENT)
 register_function(ST_AsEWKT)
 register_function(ST_MAKEENVELOPE)
+register_function(ST_CENTROID)