Widget that computes and plots parts of the Summary on-the-fly

[zblz]

Right now, creating and exploring a lens report is always a two step process: lens.summarise and lens.explore or lens.interactive_explore. It would be useful to have a single function that directly creates a widget. However, in that case we want to avoid the front-loading of computation as it would be a bad user experience. A way around that would be to generate the dask graph but only compute the nodes that are needed for each pane of the widget when entering that pane. Using opportunistic caching we would avoid recomputing all the graph up to that node as many of the previously computed nodes would be kept in memory.