Consolidate `DiscreteView` and `ContinuousView`

[milliams]

As mentioned in the design philosophy on the wiki, I'd like to be able to automatically map representations to views by their dimension types.

For starters, this means consolidating the current DiscreteView and ContinuousView into one 2D view which can have either a continuous or discrete dimension on each axis. This means that we should be able to define a view as being continuous×continuous, continuous×discrete, discrete×continuous or discrete×discrete.

The user should then be able to take a BoxPlot representation (which can be thought of as having an implicit discrete×continuous dimensionality) and map it to a continuous×discrete view so that it plots horizontally.

Before we jump into implementation, some questions:

1. Do we want to statically define the view axis types or dynamically at runtime?
2. Are continuous×discrete and discrete×continuous really different or can the transpose just be done with a boolean flag?
3. Do we need, in general, a more detailed approach to dimensions where, as well as continuous/discrete they also have a type (e.g. "city name", "height", "bin density") to make sure that only data of the same type gets plotted on the same axis?