Release

[mdsumner]

• dump the broken ARC stuff #81
• fix trip , vertices unique in x,y,t
• remove PATH from internal use (except for ear cutting, necessarily)
• document the basic cases (i.e. the model checklist issues), review the doc for sense
• fix plotting, should plot like base (black lines no fill) unless col = is specified, and then plot as primitives - use mesh_plot approach for triangles, segments() for lines
• do initial sfheaders conversions, remove build_sf and point the way

[mdsumner]

Removing PATH() was trivial because SC0 already laid the foundation.

TRI()/TRI0() should use PATH0() and not PATH(), but that's not super high-priority because ear cutting is cheap and Delaunay is required for the fun stuff.