I realized that schema migration will be more complicated with separate
subclasses which are controlled by obscore configuration. Merging them
into a single class, as I originally tried to do. It is slightly more
messier, but will work better with schema migration tools.

Obscore manager adds generic plugin mechanisms to support spatial columns
and indices. A specific implementation using pgSphere extension adds
two columns and corresponding indices that contain pgSphere polygon and
position (center of bounding circle) to obscore table.