Changes to coarsen

• improvement to independent set
• fixed bug that made it impossible to collapse edge in both directions
• fixed bug that made it slower to collapse in one direction in parallel
• added multi-step coarsen

Changes to snapping

• fixed bug with snapping coarsening the mesh too mush
• fixed bug where not all regions were included in first problem plane
• should snap vertices in most cases
• added quality settings for split collapse

More testing

• added test for common errors in coarsen, refinement and snapping

Most of the these changes to snapping and coarsen came from Li's thesis with some minor exceptions:

• In the coarsen procedure in Li's thesis it describes getting the shortest edge next to a vertex and collapsing it. We found we had better results if we try collapsing the shortest edge and then grab the next shortest until one succeeds.
• Another difference is that Li's thesis mentions collapses, but it has very little detail in how those collapses are selected, so our logic for collapses is taken from our old mesh adapt library.
• Li's thesis also describes using FaceSwap in snapping and vertex repositioning in snapping and coarsen, but we haven't created either of those operators yet.