Add Imperial Unit/U.S CustomaryUnit (currently the same) during IfcProject initialization with SI and conversion unit.

[liszto]

[ADDED] Add some common SI Unit used in all common IfcProject file. [CHANGED] Slightly change the "ProjectUnits" to add imperial in the enum and use SetOrChangeSiUnit to simplify the reading in IfcProject initialization process [CHANGED] Replace "µ" by its unicode value

[liszto]

I think it's easier to read and to setup the IfcProject in this way (with the imperial convertion stuff ) but it create a little overcost due to the logic behind "SetOrChangeConversionUnit" : few call to Linq method will create this.

Otherwise it's possible to keep your logic with the "Add" system and just add the specific calls needed during imperial creation with convertion.

[martin1cerny]

Hi @liszto , thank you for very much your contribution! We are tidying things up before we start implementing IFC4.3.