rewrite functions in pv_model

[SimonHuette]

resolves #688

I will try to optimize the beam radiation for times close to sunrise or sunset. I will adjust the case distinction of omega1 and omega2, also allowing for smaller intervalls between them. This will be balanced out by multiplying the power with the time difference between omega2 and omega1, thus not creating very high peaks in power outputs in the morning (or evening). Also, I removed the condition Theta_G < 90°, since that should only result in a negative value of r, which is already accounted for, making it unneccessary.

Please let me know what you think!