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).

Remember, at this level we should not talk about visualization options if we can avoid it.

When you say "separate color, angle and magnitude ranges" do you mean separate methods? i.e would you want ``range_dim1``, ``range_dim2`` methods  (horrible names!) or do you mean we keep a single range method that returns all the ranges at once (i.e a tuple of tuples which may be ``(None,None)`` if unavailable).

Both these options would involve changing how ranges are processed at the stack level. Although somewhat ugly, the proposed solution here integrates with the existing system with minimal effort.