add GAS_DENSITY_NORMED to unitless

- add GAS_DENSITY_NORMED to calculate 2D surface density out of 3D volume density
change unitless

- change formating of default `physicalConstants.unitless`
- update all examples `physicalConstants.unitless`
Merge pull request #445 from psychocoderHPC/topic-cleanUpDensity

add SI::GAS_DENSITY_NORMED