Merge pull request #487 in VECTO/vecto-sim from...

Merge pull request #487 in VECTO/vecto-sim from ~EMQUARIMA/vecto-sim:feature/VECTO-550-allow-custom-settings-for-airdensity to master

* commit '6f091d62':
  add attributes to air density property: required
  adapt testcases related to airdrag: pass along air density; set air density in model data in testcases
  add air density (and gravity acceleration) as constant to declaration data. add air density to vehicle data, use air density from model data for air drag computation declaration data adapter: use declaration data; engineering data adapter: use physics constant to fill model data GUI: set air density (physical constant) in declaration mode