Change how a trihexaflexagon is drawn

Change how a trihexaflexagon is drawn to obtain a hexaflexagon more
aligned with conventions used elsewhere.

Now the hexagons are drawn with a "vertical orientation", opposed to the
previous horizontal one where they looked like they were laying on the
x axis.

This shows more clearly that the triangles are grouped in couples and
these couples only change orientation on one axis when shown on the
"opposite" face.

To have a clearer view of hat that means take a look at "The symmetry
within the trihexaflexagon, with Simon Pampena":
https://www.youtube.com/watch?v=NBJrGbbxceU