Add commands for analyzing practice exercises and their prerequisites

[angelikatyborska]

To be able to add and change prerequisites and practices to practice exercises efficiently, it would be very helpful to have tools that do the following tasks:

• print all concepts that aren't required by any exercise, aren't practiced by any exercise, and maybe the difference between those two lists (concepts that are required, but aren't practiced)
• for a given concept, print all exercises that require it, that practice it, and maybe the difference between those two lists (exercises that require the concept, but don't practice it)