David/dry ue populate

[davidcherney]

This PR has three aims in changing repetition in ue_populate.py.

1. Reduce the repetition
2. Reduce the complexity of the IMSIs; the stacked if statements have been replaces with a counted integer to string.
3. Add descriptions of the arguments. This was done by breaking the arguments into their parts (e.g. IMSI = [MCC|MNC|MSISDN]) and by adding comments. I plan to move the comments from here to the openverso ueransim-ues documentation, initiating our interaction there.

To test, I suggest using the following. python ue_populate_creation.py