Add a debug flag

[jyn514]

This actually existed a while ago, I took it out because I didn't have much that fell into 'warnings for developer' as opposed to 'something is crashing the program but will work once you fix it' (and because I'm stubborn and like concise code more than useful code).

Now we have warnings again, mostly data quality in the parsing stage, these should be controlled by a verbosity flag. This is implemented in the CLI (under the parent parser verbosity) but not in the individual functions.

Warnings that should be optional:

• duplicated department descriptions
• invalid departments
• duplicated instructor emails

[jyn514]

Ideally this would use the logging module; this would reduce maintenance burden by a great deal

[jyn514]

done in https://github.com/jyn514/GradeForge/commit/c6b6bc67e588ac7aca517f13a8f4e8b36234d49e