model/airport: Use sqlalchemy cast()
model/geo: Use sqlalchemy cast()
lib/sql: Removed unused cast() "function"
model/geo: Added from_wkb() method
model/geo: Removed unnecessary ST_AsText() call

We can handle WKB directly now...
model/geo: Use new geoalchemy2 syntax for function calls
lib/sql: Renamed extract_field to PGCompositeElement

.. and added type parameter.

inspired by
https://groups.google.com/d/msg/sqlalchemy/f9BPVHfdvbg/M88ruLo6lzQJ
lib/sql: Added PGCompositeType class

inspired by
https://groups.google.com/d/msg/sqlalchemy/f9BPVHfdvbg/M88ruLo6lzQJ
model/geo: Moved GeometryDump definition into lib.sql module
lib/sql: Added ST_Dump function

This function can be applied to Geometry instances and will return
GeometryDump objects