[Developers] Update /aviation/aircraft/comparable_aircraft to a sibling CVT

[Bryan Cheung]

A suggestion has come in from a user to update /aviation/aircraft/ 
comparable_aircraft from a simple relationship to a symmetrical  
relationship (sibling CVT - see sibling relationship or adjoining  
relationship as examples).  Are there any comments/objections to this  
change?  This will break any applications that are expecting a simple  
relationship to be returned.

Bryan