fix: indexing wrt class_names & relevant issues

[4SkyNet]

Thx for your cool work wrt dtreeviz package!

But, I've recently encountered with some issues, for example:

• if you have not ordered class_names (suppose not from zero):
  np.unique[y] => [1, 2, 3]
• you get class names (if you provide them as sequence):
  {0: 1, 1: 2, 2: 3}
  and...

    for i, c in enumerate(class_values):
        print(i, c)
        box = patches.Rectangle((0, 0), 20, 10, linewidth=.4, edgecolor=GREY,
                                facecolor=colors[c], label=class_names[c])

This code cause the error

• if we manually provide dict via existing list:
  {val: idx for idx, val in enumerate(class_names)}
  We also fall in error there:

X_hist = [X[y == cl] for cl in class_values]
for cl in enumerate(class_values):
    hist, foo = np.histogram(X_hist[cl], bins=bins, range=overall_feature_range)