Air density based on temperature for #995

**WIP**: Do not merge yet. Based on the branch featured in #1002 (which probably should be merged first).

Calculates annual and daily temperature changes, and their effect on air density and hence ballistics.

Default temperature parameters based on actual data from Limnos Island (http://www.weatherbase.com/weather/weather.php3?s=5661&cityname=Limnos-Greece).

![image](https://cloud.githubusercontent.com/assets/7674951/4045247/2cec0948-2d28-11e4-8e9b-ccbb339299b0.png)

![image](https://cloud.githubusercontent.com/assets/7674951/4045258/34e4e91c-2d28-11e4-91a2-ed9d506f01d6.png)

![image](https://cloud.githubusercontent.com/assets/7674951/4045532/b5783e2e-2d2a-11e4-9716-abb751aeeec0.png)