Guessing values for the atmosphere settings

[alex-w]

Original report: https://bugs.launchpad.net/stellarium/+bug/1361709

This feature request was obtained in discussion about implementation of the extinction feature: https://sourceforge.net/p/stellarium/discussion/278769/thread/4b322044/

Current extinction coefficient, temperature and pressure for extinction/refraction just sets as parameter for atmosphere settings. We should introduce feature for guessing these parameters as function of elevation of the place, season time and size of population in this place at first step (taken some average values). On the second step we can use data from the meteo stations (e.g. in METAR format) for adjusting atmosphere settings in real time mode (plugin?).