Add support for ordering solutions by where the problem occurs in the end-of-chapter

I also added a test collection that only exercises the ordering problem

To test, I generated the `ccap-numbering.css` file so Docbook elements were hidden and problems/solutions were numbered (fewer visual distractions). Also, for testing I did `> div.cnx-eoc { counter-reset: /*exercise*/ eoc-section -1; }`

This should be tested with more content though.