Adjust GFlow to apply at deepest ceiling instead of at max depth

This moves the point where GF_low applies up which implies that at all
shallower depth (i.e. during deco) a lower GF results which makes the deco
longer compared to the previous implementation.

Of course, "GF_low" applies at first deco stop is a bit tricky since the
depth of the first deco stop again depends on GF_low, i.e. there is
another equation to solve. You can do this by inverting the equation for
the ambient pressure and use GF_low as the gradient factor. This yields

amb = (b * M_value_corrected - GF_low * a * b) / ((1-b) * GF_low + b)

Signed-off-by: Robert C. Helling helling@atdotde.de
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>