Tidy up other geom calculations in backfill/tasks.py, while we're here.
Tidy up other geom calculations in backfill/tasks.py, while we're here.

mozilla/ichnaea,therewillbecode/ichnaea,therewillbecode/ichnaea,therewillbecode/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea