Displaying scores per class

[ludovicmoncla]

For those using multiclass classification, it would be interesting if you add an example in the documentation to display the scores per class. This can be very useful, especially for unbalanced datasets.
For instance, for displaying fscore on the test set:

for label_id, label_name in annotated_images.labels_map.items():
    print(label_name, my_classifier.metrics['epoch_fscore_'+str(label_id)+'_test'])

I suggest adding this before the Saving your work section: https://mapreader.readthedocs.io/en/latest/User-guide/Classify/Train.html#saving-your-work

[kmcdono2]

Thanks @ludovicmoncla - we'll add this!

[rwood-97]

Part of #417