Feature improve dates

I've slightly improved the handling of dates:

* The values in the date dimension now have a `sameAs` property which links them to the official data.gov.uk URIs. I've done that rather than replace the current codes being used as I thought it was important to retain the original ONS structure.

* I've added a `reporting_period` dimension to the main dataset. This refers to the reporting period concept scheme (annual, quarterly, monthly). This makes it easier for us to pick out which observations in the main dataset are annual figures from those at different date periods. So, we can now fetch all annual (or quarterly, or monthly) observations for a given product separately to the rest of the time series.

Closes #6.