Add default constructor if there's no other constructor

[karimi]

All classes that don't have explicit constructors defined should get a default constructor without any arguments.

For example ColorGradient Class has a single default constructor that's missing in our implementation:
https://developer.rhino3d.com/api/RhinoCommon/html/T_Rhino_Display_ColorGradient.htm

https://mcneel-apidocs.herokuapp.com/api/rhinocommon/rhino.display.colorgradient