Extract coverage object
Department return data coverage object
Calculate department data coverage correctly

It shouldn't be the average of the service's data coverage, as the
values not provided skew the calculation. It should use the total
requested and provided
Use sum instead of reduce
Refactored total volumes to reuse methods
Fix sort by data coverage
Merge pull request #103 from alphagov/fix-data-coverage-exclusion

Fix data coverage exclusion