Prosrs

- Change the naming `true_best` to `post_best`.
- Change `min_true_func` to `min_val`.
- Add safeguards to avoid `numpy` loading errors due to inconsistent Python versions.
- Make `f` optional in the `Problem` definition.
- Decompose `posterior_eval` method.
- Add shield badges to README.
- Add one more advanced example.

- Update README in the home page.

- Improve exception handling.

- Simplify exception handling.

- Fix warning and error handling.

- Add `__version__` attribute to the package.
- Make `matplotlib` package optional.