Improvements and fixes

Performance ratio ignores frozen irradiation sensors, Janitza meters report feed-in in the right direction, and data sources recover without an agent restart.

• The performance ratio is no longer skewed by an irradiation sensor that freezes during the day while another sensor still sees daylight.
• A Janitza meter set up through the guided logger setup now reports feed-in in the right direction, whichever way its current transformers are wired.
• A data source unreachable when the on-site agent started now delivers its readings as soon as it answers again, without an agent restart.