Exposing data and summaries for each of the visualisation functions

[njtierney]

related to #78

For example, vis_expect gaining an summarise_expect function to tell you about the expectations and where they are true etc.

There could also be summaries and exposures for each of the functions - still working on names, but perhaps:

expose_vis_dat would provide vis_dat data structure before it gets fed into ggplot - this would have an S3 .grouped_df method. summarise_dat - would provide the summary of the class of each variable. Not that interesting in this case, perhaps, but interesting for things like summarise_expect, which would return a dataframe with summaries for each column, the expectation, and how frequently it was true.