removed IDeviceState.Update
It doesn't need to be part of the interface for specific implementations to have an Update method, and for ReadOnlyDeviceState, the Update method makes no sense.
refactored NodeDataEntry to cutout repeated code
keep track of when Open Z-Wave data has updated
use Describe method for MultiLevelSensors
add TimeStamp to IMultilevelSensorState
update website device examples to have TimeStamps
update Open Z-Wave MultiLevel sensors with TimeStamp