Cupid-matching

- fixed error in bias-correction term.

- corrected typo.

- simplified the bias-correction for the minimum distance estimator in the Choo and Siow homoskedastic model.

- added an optional bias-correction for the minimum distance estimator in the Choo and Siow homoskedastic model, to help with cases when the matching patterns vary a lot across cells.
- added two complete examples: `example_choosiow.py` and `example_nestedlogit.py`.