Changes

PSG: add atmospheric backpressure

Now accounts for lower ISP/thrust at the surface due to atmospheric
pressure.

In my test rocket the pad prediction was 408 m/s left at insertion
and it wound up 398 m/s at insertion (using A_ref and Cd values
from FARc)

Signed-off-by: Lamont Granquist <lamont@scriptkiddie.org>
 (commit: c62750d)