[MAINT] Add unit tests for `Geometry` module.

This fixes #1.

TODO:
- [x] Fix library dependencies.
- [x] Reorganize existing unit tests:
  - [x] Polygon testing.
  - [x] CSG
  - [x] Ellipses
  - [x] Cone
~~- [ ] Enrich tests in each submodule.~~ Too long. Address this in several new PRs.