Improved Point plots and VectorFields

This PR aims to add improved point plots and vector field plots to DataViews.

Work is ongoing on ``VectorField`` and ``VectorFieldPlot``. Right now, the PR shows only the changes to how ``Points`` are handled.

The aim is to support points displayed in sheet coordinates with either:

* No data (0D additional data) - only positions are marked.
* Scalar data in a a scalar field (1D additional data, point magnitude visualized via colour map).
* A vector field (2D additional data, visualized as a quiver plot.).

The ``Points`` class now supports the 0D and 1D cases and ``VectorField`` will support the 2D case (i.e. quiver plots).

I would prefer having separate color, angle and magnitude ranges, returning ```(None, None)``` if that dimension isn't specified.