(simple) polar projection

[Fil]

A stream transform that follows a polar coordinate system.

It's not an azimuthal geo projection: the main difference is when we interpolate between points to create a line. Instead of geodesics, we follow the circle (and use arc path instead of geoPath).

• A line that goes from 0° to 270° still goes in the clockwise direction, not taking the "shortest path" by going anticlockwise.
• The angular component of a move is along a circle centered on the origin (in spherical geo, this is identical only on the equator)

Then make Plot.area compatible with projections, and you have pie charts.

For circular bar charts and radial bar charts (category x quantity), we'll have to figure out how the projection can access the scaled categories to determine the radius (circular bar chart) or angle (radial bar chart).

cc: @tophtucker

[Fil]

duplicate of #133