added lecture_4
Merge branch 'master' of https://github.com/mas-dse/DSE200-notebooks
new Topic_Models notebook
Removing exercises until final draft
added argparse to the script at the end of the first notebook
Merge branch 'master' of https://github.com/mas-dse/DSE200-notebooks
Merge branch 'master' of https://github.com/mas-dse/DSE200-notebooks
Did 60 for forward selection. Still need to do discussion.