Collada loader units fix

This patch introduces the ability to read the unit scaling factor from Collada mesh resources. 


ASSIMP reads this scaling factor from Collada files, but does not expose an interface to retrieve this value through the Importer interface. ASSIMP may not ever expose this interface because only Collada files have this attribute. 

Previously, it was acceptable to simply ignore this attribute as nothing else used it, but the current version of Gazebo does, and so it must be used to maintain consistency.