Add non-linear multiplier2 to solar calibration

[NigelG]

I would like to suggest the addition of the non-linear multiplier2 to the solar calibration input on the settings/calibration page. This would enable a finer adjustment of the output from a solar radiation sensor or lux sensor to better match the theoretical maximum curve. The multiplier is already available for outdoor temperature and humidity. This feature would allow users to experiment with diffusers to try and obtain as good as cosine response as possible as some sensors have a poor response at low sun angles.

[mcrossley]

All the calibrations have been made consistent in the latest release - v3.25.0