Wnb

- Bug fixing and improvements

- Add mixins for probability distribution classes: `ContinuousDistMixin` and `DiscreteDistMixin`
- Restructure distribution classes in `wnb.dist` module to inherit from mixins
- Add support for custom-defined distribution classes

- Add the `BernoulliDist` class to `wnb.dist` module
- Add the `GeometricDist` class to `wnb.dist` module
- Add `BERNOULLI` to the `Distribution` enum class
- Add `GEOMETRIC` to the `Distribution` enum class
- Fix bug in `__repr__` methods of distributions' classes
- Fix bug in listing distributions in `wnb.dist.AllDistributions` variable

- Add `GammaDist` class to `wnb.dist` module
- Add `GAMMA` to `Distribution` enum class

- Add `ParetoDist` class to `wnb.dist` module
- Add `PARETO` to `Distribution` enum class

- Add `GeneralNB` class for general naive Bayes classifiers
- Add a new module (`wnb.dist`) for probability distributions
- Minor improvements in the `GaussaianWNB` class
- Bug fixing