Stop using mean() when displaying volumetrics data

We now get the last model with data from the collection.
Include a set of default type styles…

…so you can create a page containing standard elements (eg p, h1, h2,
h3 etc) and it typesets nicely
Remove .performance-platform-outer

.performance-platform-outer wasn’t doing anything we couldn’t do with
padding on the parent element
$govuk-blue on border-top and border-bottom

Replace PP $primary-colour with $govuk-blue because we are one GOV.UK
Reduce size of impact number on desktop
Merge pull request #649 from alphagov/typography

CSS refactor part I
Fixed bug in getValue that would try and do a .get on an undefined.

If lastDefined can't find a last defined model in the collection it will do a
pop() on an empty array returning 'undefined' this means we need to test that we
have a valid model before doing anything with it. if we don't we should call
formatValue with null (to get no data)

Added tests for this use case.
Merge pull request #677 from alphagov/feature/line-labels

Stop using mean() when displaying volumetrics data
Allow data source `setQueryParam` method to take hashes of attributes

Sometimes we want to set more than one property at once, without firing more than one change event. The easiest way to do this is in a single set call.
Ensure fetch of a collection completely resets the state

Retained references to selected models need to be deleted as they may no longer exist in the new collection.

Fetch needs to be called with `reset` option to ensure that old models which are no longer in the returned dataset are removed.
Use correct collection moethods instead of referencing internals

Using collections' own internal references for models is not recommended as it bypasses mutation events. By using public interface onto collections then we can ensure that events are fired, and views which are dependent on the collection contents can be updatd as appropriate.
Standardise date formats in data-source model when constructing query string

By having a single place to normalise dates we can avoid having to care about it anywhere else. We can pass dates in any valid format, and the model will magically parse them into ISO8601
Cleanup of long-winded method

This should have been done in the earlier commit/PR when it was created, but it got missed.

There's no need for the apply dance, we can just call `lastDefined` with an array, and `pluck` is way more consise.
OCD
Add basic date picker implementation
Re-render table when collection is reset

If date pickers are changed to expand/reduce the dataset, then the table should change to reflect this.
Ensure dates are always ISO8601 format

For compatability with backdrop queries, and to avoid timezone ambiguity
Clean up getPeriod methods

Most of the collection internals being referred to didn't exist. Make it a lot simpler, and draw a distinction between axis-period and period