Clean up weather inputs

[kuadrat]

Some columns are calculated by the model (snow) and others could and should be (ET0). Others seem to be altogether unused (rSSD). In all cases, there's no point in providing them as model inputs.

[kuadrat]

Upon closer inspection, the following weather related parameters are never used in the model:

• DL (day length)
• Tn (from Tmin)
• Tx (from Tmax)

And the following fields and some of them should be calculated from other inputs:

• snow from PP and Ta
• PAR from SRad: PAR = SRad * 0.47 * 24 * 60 * 60 / 1e6
• ET0 (complicated calculation)