Two gasmix patches

[Robert C. Helling]

Hi,

here are two patches conceding gas mixes in the planner:

The first removes the gasmix_is_null() test which used to test the “fo2=fhe=0 means use the last gas again” logic. I could not find a place where we actually set the gasmix with that meaning but I found several where this breaks gas handling with air (represented by the same values). 

So please test if this breaks anything!!! If it does, please use a different out of band value to signal “same gas as before” not the same as air. I propose to use fo2=-4711.

The second patch corrects the logic for choosing gas mixes when inserting waypoints: A new waypoint subdivides a previous segment. So both new segments should by default inherit the gas of the old segment. This patch implements that (it used to be the gasmix of the segment preceding the old segment which makes no sense at all. Note that the gasmix of a segment is determined by the waypoint at the _end_ of the segment.

Best
Robert

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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics  
                      Scientific Coordinator                                   
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik    
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