Add _concatenator method to GeometryBlock

Following on pandas-dev/pandas#17728

Author: mrocklin

geopandas/_block.py

@@ -10,10 +10,11 @@
 
 
 class GeometryBlock(NonConsolidatableMixIn, Block):
-    """ implement a geometry block with uint pointers to C objects
-    as underlying data"""
+    """ Pandas Geometry block with pointers to C GEOS objects """
     __slots__ = ()
 
+    _concatenator = concat
+
     @property
     def _holder(self):
         return GeometryArray

geopandas/vectorized.pyx

@@ -1566,7 +1566,9 @@ cpdef cysjoin(np.ndarray[np.uintp_t, ndim=1, cast=True] left,
     return left_out, right_out
 
 
-def concat(L):
+def concat(L, axis=0):
+    if axis != 0:
+        raise NotImplementedError("Can only concatenate geometries along axis=0")
     L = list(L)
     x = np.concatenate([ga.data for ga in L])
     return GeometryArray(x, base=set(L))