ENH: Register SQL database callbacks for factories.
If the SQL database classes are not being compiled into VTK directly, register their callback methods so that they can be registered later. This is being used in the modularized VTK source tree, the ParseURL method was made public in the derived classes. Alternatively the classes could be friends with the callback function, or the superclass could call ParseURL on the newly constructed database objects. Change-Id: Iea878da34fb37b8fdf64188e80acca3fee988718