extracting sections

[sjsrey]

This drafts out ideas on how to extract cells of different types, as well as ranges of cells.

Once we have settled on a scheme for the @include cells in the template notebooks, this can be updated. Ultimately this will move into a module not a notebook.

[jreades]

Have reverted my earlier commits so as to remove conflicts with this merge request. Will now evaluate whether I can reapply my edits.