Bell function for adding to other OpenSCAD models.
Not particularly elegant, but functional...
using the scad file, bell module:
set height to the overall height of the bell in mm
set radius to the outer radius of bell in mm
set thickness to the thickness of bell wall in mm
set ratio to the the ratio of top to bottom curves (between 0.4 and 1.75)
set centersectionlength to the length of straight section inserted between the top and bottom in mm