In theory, all interior triangles have a negative spherical excess, and all exterior triangles a positive excess.
But with collinear points on the hull, some of the triangles have an excess "around 0" and it appears more robust to keep them with the interior triangles and build the hull from that lot (at the risk of missing a few), rather than trying to focus on exterior triangles, where a missing edge destroys the hull.
fixes #37
In theory, all interior triangles have a negative spherical excess, and all exterior triangles a positive excess.
But with collinear points on the hull, some of the triangles have an excess "around 0" and it appears more robust to keep them with the interior triangles and build the hull from that lot (at the risk of missing a few), rather than trying to focus on exterior triangles, where a missing edge destroys the hull.