Add parametric numbers throughout the application

[madhephaestus]

Most CAD packages let you define a bunch of m=numbers and equations as numeric sources of information. It would be nice to

1. Define numbers with variable names
2. Define equations based on those names
3. Choose between setting a number or selecting a variable when setting numbers
4. Changing numbers regenerates the model with the new values used in the operation step

[rondlh]

That would be a great feature!