#150 Consider adding global parameters

[Kenneth MacCallum]

It would be helpful to be able to have global parameters (like variables) that can be referenced in equations. For instance, if I set up a spline that is a circle by setting the points to points on a circle, it would be nice to state the radius and maybe an offset as a global parameter. That way editing those parameters change the whole spline polygon without having to edit all the points defining the circle. This will make a project quite parametric. You could adjust the desired position of a lens assembly by changing that parameter alone.