public IVector3 RotateZ(IVector3 rotationPt, double angleRad)
{
IVector3 ptTrans = Translate(rotationPt.MultiplyBy(-1));
Rotation rot = new Rotation();
IVector3 ptRot = rot.AboutZ(angleRad) * ptTrans;
IVector3 ptOut = ptRot.Translate(rotationPt);
ptOut.Col = Col;
return(ptOut);
}