Write converters for other libraries

sklearn-onnx only converts models from scikit-learn. It implements a mechanism to register converters from other libraries. Converters for models from other libraries will not be added to sklearn-onnx. Every library has its own maintenance cycle and it would become difficult to maintain a package having too many dependencies. Following examples were added to show how to develop converters for new libraries.

• Converter for pyod.models.iforest.IForest