Made changes to Python: "Your First Script" materials

 - Changed code from python2 to python3
 - Renamed files/variables/etc from "CG content" to "GC content"
 - Added comment syntax
 - Added function comment to calculate_gc_content* files.
 - Made a few minor spelling/formatting changes.
Merge branch 'master' of https://github.com/mollygibson/bioinformatics_wkshp_washu