Fix PvModel output calculation

[danielfeismann]

The electric output calculation from PvModel seems to be partly wrong when assuming the elevationAngle as the angle between the horizontal plane / ground and the panel.

Assesment so far:

• Output of east-directed PVs are higher than expected values
• Output of west-directed PVs are lower than expected values
• elevationAngle takes all values, documentation gives just the interval from 0 to 90°
• Output of west-directed PVs with elevationAngle between 360° to 270° seems to be approx what would be expected for 0 - 90°

To do's:

• [x] Do further investigations
• [x] Find solution
• [x] Extend test methods