Enable setpoints to be used as meta PVs #120
Description
The code for meta PVs currently assumes that the addresses listed in the local config can only be PV.readbacks. We should support using PV.setpoints as meta PVs as well.
A good place to start is Client.from_parsed_config, where the client's list of addresses is fetched. SnapshotTableModel will also need to be able to return data from PV.setpoint_data and as well as from PV.readback_data.