Reuse code from period query in grouped period query

@robyoung
@maxfliri
WIP: Fill gaps in period query
Fill gaps in period query

Completes the partial implementation from previous commit.

@robyoung
@maxfliri
Fill gaps for grouped period query

@robyoung
@maxfliri
Use datutils for calculating mondays

@robyoung
@maxfliri
function to calculate week start and end

@robyoung
@maxfliri
Move time series logic to timeseries function

@robyoung
@maxfliri
Merge branch 'master' into fill-gaps-in-period-query
Enable feature test for query with data gaps

@robyoung
@maxfliri
Fail at first run that finds an error

@maxfliri
Period logic moved to class Week

@robyoung
@maxfliri
Use WEEK to calculate week start

@maxfliri
@robyoung
Merge branch 'master' into fill-gaps-in-period-query

Prior to issuing a pull request
Fix PEP8 violation
Merge pull request #45 from alphagov/fill-gaps-in-period-query

Fill gaps in period query