the greedy solver now orders which nodes it will satisfy by average preference
added a test
moved greedy solver related test into new file
removed vestiges from preconditioner tests
changed vectors to sets to prevent duplicates in commod recipe ctx
a bit more docs
fixed cycpp codegen error for set.insert
fixed iteration-erase bug in ApplyThreshold
Merge branch 'fix-crctx-again' of https://github.com/rwcarlsen/cyclus into rwcarlsen-fix-crctx-again
Merge branch 'node-order' of https://github.com/gidden/cyclus into gidden-node-order
documentation clarification on exclusive node groups