Plot decision tree

[lars-reimann]

Is your feature request related to a problem?

Decision trees are one of the few models that are easy to visualize. However, we don't offer any visualization at the moment.

Desired solution

• Add a method plot() -> Image to DecisionTreeRegressor and DecisionTreeClassifier.
• Raise a ModelNotFittedError if the model is not fitted yet.

Implementation:

• Use plot_tree.
• Avoid code duplication, e.g. by adding the method to the common superclass _DecisionTreeBase.

Possible alternatives (optional)

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Screenshots (optional)

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Additional Context (optional)

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