Fix CSGs.

[ikeough]

BACKGROUND:

CSGs for solids with surfaces that had convex outlines were broken. In the image below you can see a failed CSG where wide flange sections, which have concave outlines of their end faces, create errant CSG polygons. Bad CSG polygons resulting from concave outlines.

DESCRIPTION: This PR applies tessellation for all faces to ensure that all CSG polygons have convex faces. The PR also moves the extension method Solid.ToCSG() to be an instance method on the solid class.

Healthy CSG cutting through elements with concave face outlines.

REQUIRED:

• [x] All changes are up to date in CHANGELOG.md.

---

This change is