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);