Gets the range for the field of view for the active sphere, passes it to the optional callback function (if provided), and returns it. See sendData for how to set these values.
Parameter:
callback(function): (OPTIONAL) A callback function to execute. It will be passed an object with the following attributes automatically:cameraFovMin(number): The minimum valid field of view for the current spherecameraFovMax(number): The maximum valid field of view for the current sphere
Returns: A promise that resolves to the same object that is passed to the optional callback.