Scatter Component

[curran]

This component should form the basis of a scatter plot. When composed with the Circle component, it should form a scatter plot. It should also be possible to compose with the Pie component, to create a scatter plot of pie charts. It should be able to compose any other component in this way too (e.g. scatter plot of line charts, scatter plot of stacked bars, scatter plot of streamgraphs).

• [x] Create elements for each scatter mark
• [x] Add X linear scale
• [x] Add Y linear scale
• [x] Transform by X and Y scales
• [x] Add size sqrt scale
• [x] Use D3 Local to pass size from scatter marks to nested components (source diff in Scatter component)
• [ ] Support optional size. There should be a default size that is used if the size column is not set up.

[curran]

Depends on #7