One-line

- A bug fixed in `NeuralNetwork.iter_epoch()` and `NeuralNetwork.iter_batch()`.
- Improve the functionality of `NeuralNetwork.iter_batch()`.
- Optimization and Bug fixes in `NeuralNetwork()`.
- Add `load_weights()`, `reset_weights()`, `save_weights()` and `layers` in `NeuralNetwork()`.
- Add some fast-using functions.

- Move `io.py` and `convert.py` to dedicated packages for future functional extensions.
- Bug fixes.

- Add `NeuralNetwork.iter_epoch()` and `NeuralNetwork.iter_batch()` for advanced using.
- Remove `scipy` from requirements and add `matplotlib`.
- Add `Error()` class for universal error raise.
- Import issue fixes, bug fixes and compatibility optimization.

- Add more docstrings.
- Add compatibility function `import_optional_dependency()` for usability.
- Rebuild `MachineLearning()`, which now is easier to use.
- Remove `scikit-learn` from requirements.
- Minor bug and typo fixes.

- Add some useful features to `NeuralNetwork()`.
- Bug fixes.

- Remove `seaborn` from requirements.
- Add a fast training tools `NeuralNetwork()` based on Pytorch.
- Structure optimized.