Stackable Polygons - now more parametric

Tweaked meirm's Stackable Polygons to:

1) Make the OpenSCAD code more parametric and commented (i.e. replaced numbers with variables and equations).
2) Made the ends of the 'bones' rounded, to make them easier to slide onto the 'polygons'.

Meirm did the hard math, and the polygons were already parametric (number of slots).

Attached are STL files for polygons with 6 and 8 slots, and 'bones' length 20 mm and 40 mm.