Improvements and fixes

Power and curtailment values stay complete when an inverter or the on-site agent drops out, and device web interfaces open correctly again.

• On plants that publish their combined inverter power as the grid series, an inverter that stops answering no longer makes those values drop.
• After the on-site agent restarts, a plant no longer reports its operating state as unknown until its next state change.
• Live curtailment values from pyranometer and satellite data no longer show gaps of about ten minutes.
• The clear-sky curve no longer breaks off between yesterday midday and midnight.
• Opening a network device's web interface reaches the device's own start page again instead of an address that no longer answers.
• Combiner boxes and strings now carry the data logger they are read from, as inverters already did.