[Patch] Update altitude on unit change

[Robert C. Helling]

Hi,

when changing to imperial units, the altitude got the new ft suffix but kept its numeric value. Now that is adopted to the new unit system as well.

As I learned on IRC, mbars as surface pressures are even used by imperial folks (and not PSI).

Fixes #681

Best
Robert
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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics  
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