Add method to delete a source

[dr-rodriguez]

It would be useful to have a method to completely delete a source and its associated data. While the present of triggers or foreign keys with "on delete cascade" can make this trivial, not all databases will have this. This method can work similarly to db.inventory() in that it grabs all of the data and displays it to the user. If the user agrees that this is the data to delete, the script can then go through the various tables deleting by their id columns and ending by deleting from sources.