I only called it Uri because that's what the "format" is in the schema and that's what the type is on the .NET side of things.  Whichever you guys or @shunter wants it fine with me.  I think we need to standardize on stuff like this for CZML2 (for example, right now `image` has a sub-property `image` rather than `uri`.  This means the JSON object is `image.image` rather than `image.uri` which is weird.