surface: Make Aerial object take effect of nearby RoundAstrobj

Also corrected SurfaceWar::atmosphericPressure() to return air pressure
by distance to nearby RoundAstrobj instead of uniform z-increasing
function.
Add a mission to Mars to demonstrate surface gravity

Also, Mars surface is modulated by simplex fractal noise.

Unlike Ceres, an Aerial vehicle can fly around Mars because it has
atmosphere (although thin) and has higher gravity than Ceres.