Add length and vector subtraction methods
Add Volume calculation
Fix bug in rhino to plankton mesh conversion

Boundary vertices weren't always being given the correct outgoing halfedge (which should be a boundary halfedge)
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