Rolling 7 day average

[isaacmg]

Based on @wwymak comments we would like to try forecasting cases on a rolling weekly average. While this removes a degree of granularity it may ease problems with reporting issues.

Acceptance criteria:

• [x] Experiments logged on primary counties list.
• [x] Pre-processing code for merged into the task-ts repo
• [ ] Report detailing finding of using the seven day average.