Feature date dimension relations

* Adds `broader`, `narrower` relationships. This means there's a now a hierarchy from year -> quarter -> month
* Added `previous` and `next` relationships. These are qualified by the period as described in #17. So for a quarter may have a previous relationship to the previous quarter, but will also have a previous relationship to the same quarter last year. This allows moving forwards and backwards between dates as the same level in the hierarchy
* Adds a `type` property to the dates, indicating whether they are a `year`, `quarter` or `month`. This makes it easier to find all values of a particular type, which will be useful for slicing, e.g. all years for a given product
* Removes the `reporting_period` dimension as the same information is now captured in the scheme. 

Closes #17