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

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](http://dask.pydata.org/en/latest/caching.html) we would avoid recomputing all the graph up to that node as many of the previously computed nodes would be kept in memory.