Customizable Curve Generator, updated

This is exactly the same as [the original thing 50249](https://www.thingiverse.com/thing:50249) except that this source removes use of the now deprecated `assign` function in OpenSCAD. This is so that it will continue to function after `assign` is removed from OpenSCAD at some point in the future. I have made no other changes to [kakaroto](https://www.thingiverse.com/kakaroto)'s excellent work; I just wanted to preserve it for the future.