Collecting tracks by "producers' producers"

producers_producers_tracks.py collects all tracks by all those
producers who are followed by producers in a database of collected
SoundCloud data (for example, collected using the rand_samp.py
program). Collecting all tracks by producers followed by EVERYONE would
require a vast number of API calls and is not necessary for our current
methodology.

Contains mechanisms for restarting after a crash, automatically backing
up, and restarting from a backup.
List of producers' producers

List (with Unix and Windows line endings) of users to be collected
using the producers_producers_tracks.py program.