Replace dead items with unset ones

* use unset items to init
* more robust circulators
* reset ToRhinoMesh/ToPolylines
* update unused item traversal tests
* make item constructors internal
* Vertex compact helper becomes CullUnused
Merge branch 'mesh-ops-will' into mesh-ops-daniel
Update mesh volume and edge length calcs w.r.t. unused items

* no need to check for dead faces in volume calc
* add unit test for volume calc
* replace dead check with unset check in edge length calc