Implement Summary Functions in Data-Frame

[Symbolics]

This issue also appears in Lisp-Stat/data-frame#4, repeated and expanded here because the lisp-stat repo is the 'landing point' for most people.

Lisp-Stat version 1.0 had a summary function that did basic formatting of a data frame. When designing version 2 of data-frames we looked at how R implemented summary functions and found that there were several packages for summarizing a data frame. There's an article on a few of them. When we looked at how the Tidy guys did it, the consensus was (basically), 'there isn't a summary that will make everyone happy' (see the thread A better summary function)

So what we decided to do in data frame 2.0 is not try to write a fancy summary function. Instead, we focused on improving the underlying summary data structures so that it would be easier to build things like skimr or summarytools.

This issue serves as a suggestion to anyone that would like to take up the task of creating a nice summary package along the lines of the ones available for R. This will be a good way to learn the basics of data-frame, and a good way to learn common lisp, if you don't already know it.