Add ST_Scale.

Note: In GDMS, we had the function
        ST_Scale(geom, scale)

In PostGIS, we have the functions
        ST_Scale(geom, xScale, yScale)
        ST_Scale(geom, xScale, yScale, zScale)

I.e., in GDMS, we assumed xScale=yScale. I decided to undo this
unnecessary coupling and also allow the user to scale the z-coordinate.
This amounted to conforming to PostGIS.