Support handling of unknown atom types

[linas]

Per discussion in opencog/atomspace#2787 (comment) if a RocksDB 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 RocksDB tables will need to store the atom inheritance hierarchy, which they do not currently do.

Perhaps this could be best achieved by formally defining new atom types in atomese!? (and then just storing the atomese!?)

See also opencog/atomspace#2789