convert correspondence.css to .scss, add itemGroupsExample.html
fix up styling for itemGroupsExample
span.output-marker
interleaved example in comment
rename correspondence-examples.js to correspondence-configuration.js
setupInterleaving skeleton
move selection functionality into ElementSelection class
remove itemIdDataAttribute and structureElementSelector
(revert temporarily to hard-coding)
utility functions indexItemByItemId, createStyleTarget moved out of prototypes
function StructureGroup with method initializeStructures
re-hard code itemSelector = "[data-id]"
indexItemsByTheirId and linkSiblingsAndCousins
add interleaved checkbox
use label "for" attribute for interleaved checkbox label
setting up interleave data
setup group ids
structureClassAttributes
interleave works (but not yet "uninterleave")
uninterleave
css & layout for interleaving