code: function ParametricGeometry( func, slices, stacks ) {
  
  	Geometry.call( this );
  
  	this.type = 'ParametricGeometry';
  
  	this.parameters = {
  		func: func,
  		slices: slices,
  		stacks: stacks
  	};
  
  	this.fromBufferGeometry( new ParametricBufferGeometry( func, slices, stacks ) );
  	this.mergeVertices();
  
  }
docs: @author zz85 / https://github.com/zz85
  @author Mugen87 / https://github.com/Mugen87
  
  Parametric Surfaces Geometry
  based on the brilliant article by @prideout http://prideout.net/blog/?p=44
   ParametricGeometry