Fix #3258 (Frechet distributions are incorrect)

I've made changes to correct problems with a number of related distributions in `stats._continuous_distributions`; discussed in #3258.

- Renamed `frechet_l` to `weibull_max`;
- Renamed `frechet_r` to `weibull_min`;
- Wrote new `frechet` (differs from both `frechet_l` and `frechet_r`);
- Reinstantiated new `frechet_l` and `frechet_r` as deprecated versions of `weibull_max` and `weibull_min`;
- Made suitable changes to documentation.