Also report when negative air pressure is encountered, because that would...

I encountered multiple cases where users accidentally have pressure in hPa instead of Pa, which propagates through MeteoIO and can cause negative pressure values in the Sun object. This causes a floating point exception, which may not be easy for users to debug and uncover.

So I propose just an additional check on pressure and throwing a proper error message. Made a merge request, not to interfere with the current ongoing release process.