Implement summaries

[jhclark]

Implement summaries, as discussed in the tutorial.

This will allow users to quickly summarize results so far in a workflow and make papers for tables, etc.

[dowobeha]

@jhclark, are the syntax and semantics for summary blocks finalized?

[jhclark]

I think basic summaries look pretty good now: https://github.com/jhclark/ducttape/blob/master/tutorial/05-01-hello-summaries.tape.TODO

However, once we add switch-case to the mix along with tasks that might have different code blocks depending on the realization, things will get nastier: https://github.com/jhclark/ducttape/blob/master/tutorial/05-02-hyper-summaries.tape.TODO. I'm not sure what to do about that yet. Or if we should be worrying about it yet. I'm inclined to think about syntax now though, so that we don't paint ourselves into a corner.

[mitchellgordon95]

The "using" keyword for specifying a specific branch for a summary was never implemented, correct?