Segments of straight line flows that are edges of some polygon

[videlec]

The behavior of segment that are edges is a bit confusing at the moment

sage: from flatsurf import *
sage: S = translation_surfaces.mcmullen_L(1,1,1,1)
sage: traj = S.tangent_vector(0, (0, 0), (1, 0)).straight_line_trajectory()
sage: traj.flow(1)
sage: s = traj.segments()[0]
sage: s
Segment in polygon 0 starting at (0, 0) and ending at (1, 1)

Here the string representation is wrong: it is ending at (1, 0). The reason is that (0, 0) implicitly refers to coordinates in polygon 0 while (1, 1) refers to coordinates in polygon 1. This becomes apparent when looking at the start and end of the segment

sage: s.start()
SimilaritySurfaceTangentVector in polygon 0 based at (0, 0) with vector (1, 0)
sage: s.end()
SimilaritySurfaceTangentVector in polygon 1 based at (1, 1) with vector (-1, 0)

[wphooper]

It is probably not well documented, but this was purposeful. When you create a tangent vector along the common edge of two polygons, it chooses the polygon that lies to the left of the tangent vector.

[videlec]

That is fine, but the string representation is still wrong.

[videlec]

And more dramatically s.end() belongs to a different polygon.

[videlec]

To give some context, my complaint comes from #308 where the case of segments has to be considered separately exactly because of this.