[Data-modeling] Proposed changes to Airport type

[Bryan Cheung]

Currently, ICAO and IATA airport designators are topics linked to  
airports.  I'd like to modify the airport schema to have ICAO and IATA  
designators as property enumerations and get rid of the types / 
aviation/icao_airport_designation and /aviation/ 
iata_airport_designation.  This way we can enforce designator  
uniqueness across airports (The IATA SFO can only be linked to the  
topic for San Francisco Airport, and nothing else).

Any objections to this change?

Bryan