[Lab 4.3] File Systems: Checkpoints, Hints

Closes #799 Closes #902

Changes proposed in this pull request:

Made test cases, input files for the two checkpoints in the File Systems Lab
Added starter code
Wrote answer code
Changed parts of networkx functionality so that code was runnable and added those changes to appropriate lab files
Restructured lab by switching 3.md and 4.md
Stylistic changes to lab
Changed lab to implement new hint format

@sarahg500

bitprj / curriculum