Add limit to the grid-generating functions

This adds an optinal maxcells parameter to the rectangle and hexagonal grids,
with a default value of 262144 (2^18).
An error occurs when the grids would need more cells.

test/CDB_HexagonTest_expect

@@ -1,2 +1,5 @@
 9|63|count / npoints
 #160|t
+3886
+ERROR:  The requested grid is too big to be rendered
+CONTEXT:  PL/pgSQL function cdb_hexagongrid(geometry,double precision,geometry,integer) line 80 at RAISE