New module: stereonet for plotting Schmidt/Wulff stereonets (structural geology)

**Description of the desired feature**

Structural geologists routinely plot fault/fracture planes and lineations on a **stereonet** — the projection of one hemisphere used to display the orientation of planes (as strike/dip or dip direction/dip) and lines (as trend/plunge). GMT can already produce the *net* itself with the existing azimuthal projections (`-JA` for the equal-area Schmidt net, `-JS` for the equal-angle Wulff net, see the Cookbook figure `GMT_stereonets`), but there is no built-in way to convert structural measurements into the coordinates those projections expect, or to draw the cyclographic trace of a plane.

This was discussed at length on the forum: https://forum.generic-mapping-tools.org/t/plot-a-schmidt-stereonet/6067

**Status**

I'm working on this — a new `geology` supplement with a `stereonet` module
(`psstereonet` in classic mode). Current scope:

- `-JA`/`-JS` (Schmidt/Wulff net), centered on 0/0, reusing GMT's existing
azimuthal projections rather than inventing a new one
- `-T` selects what the input angles mean: strike/dip (right-hand rule),
dip direction/dip, or trend/plunge of a line; `+u` for the upper hemisphere
- Draws the cyclographic trace of each plane (`-W`) and/or the pole or line
(`-S`), colorable via `-C`
- `-A` for the azimuth ring around the perimeter
- `-M` to dump the converted lon/lat instead of plotting, for building custom
figures

Opening this issue mainly so the community is aware work is underway, and to
get early feedback on naming/scope/options before a PR — happy to hear
opinions, especially from other structural geologists.

**Are you willing to help implement and maintain this feature?** Yes