Add ST_InteriorPoint

Note: PostGIS calls this ST_PointOnSurface. See also
ST_Point_Inside_Circle.
Add ST_PointsToLine

Note: This is called ST_MakeLine in PostGIS. There are three versions:

    1. geometry ST_MakeLine(geometry set pointfield);
    2. geometry ST_MakeLine(geometry point1, geometry point2);
    3. geometry ST_MakeLine(geometry[] point_array);

For now, we only have the equivalent of (1) on (Multi)Points. For
MultiPoints, PostGIS has a separate function:

    geometry ST_LineFromMultiPoint(geometry aMultiPoint);
Add ST_ToMultiPoint

Note: The only way this seems to be possible in PostGIS is via

    SELECT ST_Multi(ST_Union(the_geom)) FROM myTable