Polliwog

- Update vg dependency (now requires 1.0.0)

- Replace most np array coercion with shape checking.
- Use pint for unit conversion.
- Remove most modules from \_temporary.
- Fix erroneous in rigid_transform.

- Rewrite `polliwog.tri.barycentric` with new function signature.
- Improve test coverage.

- Fix `pip install polliwog`.

Initial release.

[entente]: https://github.com/lace/entente