Tree visualisation

[thesps]

Add Decision Tree and Model draw methods to visualise tree structure showing:

• feature and threshold for non-leaves
• score/value for leaves

The plotting uses Graphviz and pydot for drawing, which are not added to the dependencies, but will not break import if not available, and print a helpful message if drawing is attempted without them.

Here's an example for one tree (e.g. conifer_model.trees[0][0].draw(filename='tree.png')):

and here's an example for a model (e.g. conifer_model.draw(filename='model.png')):

[thesps]

Tests pass ✅