Physics update for pressure

There should be much more options for pressure in blender. The scene should contain right next to gravity options, toggleable options about the air (pressure (for cloth pressure to work correctly), density (for lift), viscosity/drag). Also the cloth simulations options for air pressure should get updated to use the scene pressure from the outside instead of using negative pressures. Also there should be a compressibility option for the cloth air pressure.

Currently there is not even a pressure unit in blender. I propose to use the unit hPa (hectopascal) or bar (= 1000hPa, because usual pressures are around 1 bar and blender likes number with minimal significant digits)