return a PMO_MyTable object from th PMO_MyTable directory or instanciates it if it does not exist yet.
factory() first checks to see if the wanted PMO_Table has already been registered. If so it returns it immediately.
Otherwise factory() will try to load the corresponding class from the PMO_MyTable/ directory.
If it does not exist, factory() will finally instanciate a generic PMO_Mytable object, get the table structure and persist it on disk and register it for the next instanciation.