Deep gas-giant atmospheres

As you might have noticed when scooping a gas giant, its atmosphere is ridiculously shallow and has an insanely high pressure gradient.

The reason is that all atmospheres in the game use a the gas parameters of air (i.e. nitrogen/oxygen). However, gas giants have a hydrogen/helium atmosphere which is very different from air. The specific heat of such a mixture is 13 times as high as that of air and the molar mass is approx. 1/13th. Just using these parameters instead of the parameters of air give satisfactory results. So, this PR uses two sets of parameters, the "hydrogen/helium" parameters for gas giants and the "air" parameters for other planets/moons/asteroids.

The results for some gas planets in the solar system are:

* Jupiter: 1 atm at 58km altitude, fuel scooping pressure approx. 8.6 atm at 38km
* Saturn: 1 atm at 104km altitude, fuel scooping pressure approx. 7.0 atm at 57km
* Neptune: 1 atm at 50km altitude, fuel scooping pressure approx. 3.75 atm at 37km

You might notice the higher pressure for fuel scooping. The game currently uses a fixed gas density of 1 kg/m³ which results in a much higher pressure for hydrogen/helium than for air. But the height is still much above the "surface", so I think it is okay (but can be changed easily, since only gas giants are scoopable).