Fco volumetrics cleanup

- Add feature tests for FCO volumetrics graphs
- Add tests for volumetrics collections
- Extract the most recent week from the backdrop response instead of assuming the date on the client side will be correct
- Factor the parse method into smaller methods, one for each series we want to represent
- Pull code for dealing with time series into a mixin
- Pull some more code up into the closure (hopefully we can remove this when backdrop is updated)
- Inherit from `Collection` and not `GraphCollection` for the volumetrics collection
- Calculate mean, total and number of weeks with data values on the collection and not in the views