Dupont-contraction

Implemented cubical Dupont and Sullivan forms with the associated Dupont-Getzler contraction.

Fixed a bug in `__repr__`.

The classes `DupontForm` and `SullivanForm` now import from `dupontcontraction.simplicial`. This is in preparation for the cubical version (to appear in a future release).

Minor changes/bug fixes:

- Fixed sign error in a_infinity_product.
- Improved README.md

No code changes, release to generate DOI.

First production ready version of this package.