Cleans up regional stats code

A lot of the code was mostly just adding complexity to simple method
calls, and hiding things that the controller should be doing.

This commit is only half way -- next up is moving the stats bits from
the controller in to a stats presenter.
Clean up Country stats library

As with prior commit -- move most of responsibility from Stats modules
to models. Most of the stats are to do with counting relations, etc. so
the relations are handled by the model, and the decision to count is
handled by the controller.
Refactor global stats
Refactors stats controller code in to presenters

Previously the stats controllers had quite a lot of say in how the stats
were presented (e.g. deciding whether or not to round the values, etc.).
This responsibility has moved to the `StatisticPresenter`.

StatisticPresenter proxies methods to the model or model statistic, and
presents the data differently depending on what was requested. For
example, the global pa coverage percentage is rounded to one significant
digit.
Update PA percentage test to reflect presenter
Merge pull request #89 from unepwcmc/stats-refactor

Stats display refactor