Update classification docs per Chris' comments

docs/tutorial/3_classification.md

docs/tutorial/classification/c_1.png

docs/tutorial/classification/c_10.png

docs/tutorial/classification/c_100.png

docs/tutorial/classification/c_1000.png

docs/tutorial/classification/data_class.csv

docs/tutorial/classification/sisso.json

docs/tutorial/classification/sissopp.png