Issue15 tmy3

Hi Ruben,

i tested the new branch on a model of mine.
I needed to make some minor adaptations from the original pull request but now it's working.

What's strange is that running the same model, I get the following:
![image](https://cloud.githubusercontent.com/assets/2987659/2916916/71ac5a92-d6c0-11e3-87a5-1350064715a2.png)

So the temperature change in the north zone is inversely proportional to the solar gain in the zone.
Any idea why?