v0.6.0

• Ensure that the population property, if present, is an integer.
• Add IATA short (3-character) codes to airports.
• Interpret road kinds for pistes, motorway junctions, racetracks and piers.
• Normalise is_link, is_tunnel and is_bridge so that it is not present in the negative; it should only be present when its value is positive.
• Re-order aerialways to be above all road types, including bridges, unless overriden by a layer property.
• Added sort order properties for beaches and winter sports areas.
• Added a function to remove abandoned pistes from the output.