Allows us to put multiple charts on a single page, where theme settings apply to only some of them

This is the same as Highcharts currently uses.
Avoids having to create svg and dom elements to get the styles.
Requires that the default style variables are set in the perspective-viewer layer.

 Put color styles in settings so that they can be scoped to a chart

Implemented a special interpolator to connect up multiple two-color interpolators
Heatmap color has to be reapplied because the d3fc one forces it to the data extent rather than the color range you've given it
Found some d3 color utility functions

Implemented themable color ranges for heatmap and X/Y

update screenshot results with new colours

Check whether labels are outside of window bounds, and hide them.
Also added support for fully rotating (90 degrees) when there are a very large number of labels, similar to Highcharts.

Hide labels that would be clipped by the window bounds

Update develop from remote master

Fix/use get config

* Refactor axis to use a common implementation for x and y

Refactor to group by type of axis (linear/time/ordinal) rather than by direction.
Use a common implementation for each chart for creating an axis, based on the settings and data it will be working from.
Allow for aggregates that are strings turned into numbers (e.g. `count`) or left as string (e.g. `dominant`)

* Allow charts to say which axis types they can't use

* Applied new axis types to other charts

* Rename files

* Refactor `labelFunction` and remove `crossAxis`

* Fix variances in screenshot tests

* Restore heatmap vertical gridline

* Exclude "linear" ranges from various charts

* Default padding for chart axis, which can be overridden

* Restore label rotation/hiding code that was lost in merge

merge view.config fix

Test/sunburst integration

* Use a "nice" number range for display purpose only

* Improved colour-legend display

Cleaner numbers (reduces display of fractions), and better display of "clean" range
Give the colour legend a bit more height, but less width, which it doesn't need

* Make legend components non-global

Prevents legends from different charts interfering with each other
Stores `pageIndex` in settings to preserve page position

* Tweaked sizing code for sunburst charts`

Leads to large charts with more space usage, when divided into columns

* Reduce scrolling if row height is "close enough"

Allows for different chart types to have different size colour legends. Heatmap has space to have a taller one, but that it not so appropriate in sunburst

Match arc opacity

Sunburst centre now shows current level instead of parent

sunburst - add labels, don't display if no crossValue is supplied

update screenshots

d3fc_plugin - sunburst chart, color changes and bug fixes

Previously, it would allocate 3 or more columns (space allowing), even if there were only two charts to show.

Allow for 2-column display when only 2 charts (#151)

IE uses left/right instead of x/y and different format "transform".
Use "closest" (with polyfill) to navigate up to the chart element.
Polyfill "matches" which is used by d3fc
Move default theme variables to where they actually need to be (IE/Edge don't get the variables otherwise)

Fix occasional integration test failure and some IE issues

Feature/parameter names

Merge latest changes