Fill in missing periods

- Fill in all missing time periods in all groups.
- Keep time series in chronological order.
- If a time period is missing from every group it will not be added.

@robyoung
@pbadenski
Fill in gaps in period groups

Fill in gaps in period groups even if period is not present in any
group. This way we always return a continuous list of periods.
Merge pull request #30 from alphagov/fill-gaps-for-period-query

:cake: