1D Plots

This is the start of a general infrastructure for plots which only distribute their data in one dimension.

The motivation are things like plotting tops and associated descriptions in geophysics files, but also allowing things like axis scatters as used by Tufte.  Potentially can move jitter plots and colorbar plots into this infrastructure.

This is not yet ready to merge.
Breaking backward compatibility was deliberate in this case, particularly as the `JitterPlot` already had `index_mapper` that is the same object and which is supported.

If the JitterPlot was widely used, I'd be much more worried about this, but given the state of the codebase and project I don't want to mess up a clean API with backwards compatibility support for a minor feature.

This PR is probably of sufficient magnitude that it maybe should even be part of a major version bump - we are adding a significant new capability to one of the principal objects int he API.