Working Cone, Orientation, Ellipsoid, Radii, and Path

[Carl4]

I've implemented working versions of the classes in the subject. Also implemented a little helper function, class_property to handle when an object property should be a specific instance of a class. It codifies repeated getter and setter codes as something like a decorator.

[cleder]

I am not happy with the change:

• return self._epoch.isoformat()
• return self._epoch.strftime('%Y-%m-%dT%H:%M:%S.%fZ')

in class _CZMLBaseObject(object):

it breaks existing test.

also I would appreciate if you could add some tests for the new functionality