Postgres handling of unknown atom types

[linas]

Per discussion in #2787 (comment) if a postgres database holds an atom type that the C++ code does not know about, then it should be auto-created in the atomspace. To make this possible, the postgres tables will need to store the atom inheritance hierarchy, which they do not currently do.

See also opencog/atomspace-rocks#6

[linas]

Closing, obsoleted by #3018