Geodesic hemisphere with regular polygon equator

_(Updated February 2022 with several improvements and simplifications thanks to [benjaminblankertz](https://www.thingiverse.com/benjaminblankertz/designs))_

While using [Jamie_K's geodesic icosahedral sphere](https://www.thingiverse.com/thing:1484333) in OpenSCAD, I ran into some issues:
* A sphere based on an icosahedron doesn't have an equator, so it's difficult to match other polygonal shapes (cylinders etc.) to it.
* Even if you cut the icosahedral sphere exactly in half, you do get a regular polygon but the diameter is slightly smaller than the sphere diameter.
* An icosahedron is limited in the number of sides available in the polygon equator if you cut it in half. At a minimum, an icosahedron cut in half has a 10-sided equator. Subdividing it, you can get 20 sides, 40 sides, 80 sides...or `5*2^(levels+1)` sides, where `levels` is the number of subdivision iterations (with `levels=0` being the original icosahedron).

Because what I really needed was a hemisphere anyway, I