Catlearn

- Gradients added to hyperparameter optimization.
- More features added to the adsorbate fingerprint generator.
- Acquisition function structure updated. Added new functions.
- Add some standardized input/output functions to save and load models.
- The kernel setup has been made more modular.
- Better test coverage, the tests have also been optimized for speed.
- Better CI configuration. The new method is much faster and more flexible.
- Added Dockerfile and appropriate documentation in the README and CONTRIBUTING guidelines.
- A number of minor bugs have been fixed.

- The first stable version of the code base!
- For those that used the precious development version, there are many big changes in the way the code is structured. Most scripts will need to be rewritten.
- A number of minor bugs have been fixed.