Adding two graphics on first quarter GPA
Added a 9th grade retention subset
Refactor with more flexible arguments

Added enrollby, exitby, gap, enroll_date, and exit_date parameters.
Adjusted the type checking in lines 18-50 to check the user supplied
arugments for types and set enrollby and exitby values when they are
the wrong type or missing. Adjusted the function to use these values
throughout.
Added two first quarter GPA result graphics