a tool to explore/manipulate data frames

[simonpcouch]

I'm curious whether we can implement a tool that would safely allow models to explore data frames more dynamically. I've been feeling that the "env" subset of tools is a bit lacking compared to the others and I think this would notch in nicely.

One of the tools from querychat is a good example of this; read-only access using only SELECT SQL statements. I think it'd be nice if the models could write in dplyr/tidyr instead, here—with the hunch that the tool will be easier to deploy if it's "all R" and easier to read by common users than if it were in SQL—though I'm short on ideas of how we could do so without opening up the "run arbitrary R code" path.

[simonpcouch]

We'll wait to explore #56 and friends further until after the first CRAN release.