Improvements and fixes

Each value read from a data logger can now be given its own factor and limits.

• Under "Advanced settings", a factor and a high and low limit can be set for each value read from a data logger.
• Inverters, combiner boxes and strings that have never sent a measurement are now counted separately, no longer as producing.
• An inverter, combiner box, irradiation sensor or feed-in meter added by hand can now be deleted from its table, after a confirmation.