Scripts in the IPoid import process pass some variant of the filename between themselves and are oftentimes transforming them after they get them. Some want just the date (YYYYMMDD), some want the filepath ($DATADIR/YYYYMMDD.jzon.gz) and it's very probable that at least one wants just the filename (YYYYMMDD.json.gz). All of these transformers are done independently by each file when they probably should be done from a single utility function.

Acceptance Criteria:

• new utility function that can take some form of the filename and return the other forms
• tests for the function
• wherever possible (preferably everywhere), the utility function is used instead of being done ad hoc by the script