Content Preserving Group By

We've done this a few times, so we'll round them up here and find the generality.

• https://github.com/timrdf/ieeevis/wiki/COLD-2013-Supplemental-Material
• https://github.com/timrdf/ieeevis/tree/master/data/source/lodcloud-tw-rpi-edu/lodcloud-wave-of-arrivals/version/2014-Apr-30

And now, https://github.com/timrdf/ieeevis/blob/master/data/source/ieeevis-tw-rpi-edu/IEEE-VAST-Challenge-2008-mc2-reverts-social-tally/src/group-by.xsl

The key thing about the aggregation is that we don't want to lose the members of the count. So, we associate the count to those that were counted using sio:has-member.