Add properties getter

AreaDensity => Add getter

Add AreaDensity function extension

Add function(double, AreaDensity, double, AreaDensity)

Remove Conversion Interface

Remove Conversion Interface because every unit conversion has its own conversion method

Add conversion interface

Add conversion interface and migrate all UnitClass to implement it

Initial Release

Add new Feature :

• Single
  • Weight Unit
  • Area Unit
• Compound
  • Weight per Area Unit