Some fixes in PvModel and test

[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!

EDIT SP: Fix PvModelIT after #1089 is merged

[sebastian-peter]

I also have two more points regarding documentation (readthedocs):

The link to source for Iqbal.1983 has become invalid. Could you find a new link here (could just be a link to the respective page of the publisher, if nothing else is available)? Also please check all other links.

Also related to literature: Could you also indicate page numbers for each source? This has already been done for some sources, but is missing for others. It can speed up understanding the model a bit.

[sebastian-peter]

I just finally finalized this PR, have a look if you like @jo-bao @danielfeismann @staudtMarius

[danielfeismann]

Thanks a lot to both of you for working this out and double check this!

I have some minor suggestions and pushed myself some little changes. Please have a look @sebastian-peter - just ideas, I don't insinst on them.

[sebastian-peter]

In terms of functionality, only the calculation of diffuse radiation on the sloped surface (the cloud index in particular) is adapted.

On the back of an envelope, these are the changes to be expected:

Using the beam radiation on a normal surface instead of on a horizontal surface increases the beam radiation in the morning and evening (when the zenith angle theta_z is large, and thus 1/cos(theta_z) is large as well). In total, this increases epsilon in some cases in the morning and evening (compared to before).

The cubic theta_z in the numerator and denominator is now done on angles in degrees, thus drastically increasing, especially for large theta_z (again, in the morning and evening). Beforehand, the term 5.535 \cdot 10^{-6} \cdot \theta_z^3 was largely insignificant, because angles in radians were pretty much canceled out by 10^{-6}. Now, the term approximates a value of 4 for theta_z = 90° (morning/evening), which then decreases epsilon values in the morning and evening (compared to before).

In total, the changes work in opposite directions, partly cancelling each other out in some cases, while leading to different results in other cases (e.g. for intense diffuse radiation in the morning/evening).

[sebastian-peter]

The largest difference for PvModelIT is this day, where still similar maximum values are reached.

[danielfeismann]

Thanks for fixing these issues!