Improve naming of argument in numberListFormatter
Add function to calculate linear ticks

This is carried over from Data Insight.
It performs two tasks that are closely related, calculating the tick
step and calculating the tick values. This can also push out the extent.
Use new tick function for total applications graph

Uses the calculateLinearTicks function to generate ticks for the total
applications graph. This allows all tick values to be passed into the
numberListFormatter that in turn allows it to better decide how to
format ticks. An example of a bug this fixes is for the following tick
values: [200, 400, 600, 800, 1000]. Based on just the extent the result
would be [0, 0, 1k, 1k, 1k] when it should be [0.2k, 0.4k, 0.6k, 0.8k,
1.0k].
Merge pull request #8 from alphagov/improve-linear-ticks

Improve linear ticks