Session04

Add 'dict_set.py' file for dict lab assignment.
Add 'trigrams.py' file for trigrams assignment.
Add 'safe_input.py' for exceptions assignment.
Add 'mailroom.py' for mailroom madness assignment.

Consider moving this `if` statement. Currently, the user can only return to the main menu by typing "return" if it's the first thing they do. However, if they instead type something else that's not convertible to a float, then they are stuck in the below `while` loop until they enter a number.