Updated GC content exercise to use command line arguments instead of
STDIN
Made changes to Python: "Repeating yourself" materials
 - Changed code from python2 to python3
 - Added function comments
 - Added hints to translate exercise
 - Added for and while loop syntax
 - Added content to for and while loop section
 - Fixed small bugs in code snippets, e.g., initializing variables
 - Made a few minor spelling/formatting changes.