Improvements and fixes

The on-site analysis now reports a string as defective, not an outage. Open string investigations now close after three days without usable data.

• In the on-site analysis, a producing string below its neighbours on three sunny days is reported as defective.
• A stuck reading no longer raises a false "Measurement conflict" in the on-site analysis.
• An event for a defective string names the inverter, combiner box and string it concerns.
• A gradually failing meter no longer appears as an outage in the on-site analysis.
• The on-site analysis now includes a plant-level reading for whether the plant's network is up.