UpdatingPV class

1. Merge branch updating-pv from RAD-Student
2. Explore classes UpdatingPV and TestUpdatingPV
3. Understand the difference true vs false in the second argument of the UpdatePV constructor.
4. Switch to UpdatingPV instead of new Channel in Cavity for amplitude and phase.
5. It will not directly work for the waveform. Modify UpdatingPV to handle array as well as a single value.