Improvements and fixes

The string analysis now runs in more situations: without a panel datasheet, on cloudy days, and where a string's orientation cannot be measured.

• Without a panel datasheet the analysis still finds each string's wiring, connections and orientation, and only the peak power is missing.
• A string whose own orientation cannot be measured now takes the orientation of its inverter, instead of being left out.
• When there are too few clear days, cloudy days now help work out orientation, counting for less than a clear day.
• A day when the plant stopped sending for an hour is still measured, instead of being set aside as overcast.
• If no strings are set up for the inverters, the analysis now says so instead of failing with an error.
• Expected production now follows the configured strings even when some lack a peak power or an orientation, instead of the plant's peak power alone.