Calc rho should use max(0.0, height+s.set.height_gnd)

[1-Bart-1]

calc_rho(s::AM, height) = s.rho_zero_temp * fastexp(-(height+s.set.height_gnd) / 8550.0)
gives bad results with negative heights.

[ufechner7]

Negative heights don't make sense. So what would you expect? An exception?