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setTransfiniteAutomatic() automatically sets the transfinite
attribute on curves, surfaces and volumes when possible. The
objective is to automate a task (setting transfinite) which is
very tedious on large CAD models (hundreds of curves), and to
help automatic quad meshers by preprocessing the trivial patches.
The number of points on curves is automatically determined according
to the sizing constraints (same number as when generating the vertices
on curves). The number of points is kept coherent between adjacent
quadrilateral surfaces by computing the topological chords where
possible, and using the averaged values on the independent chords.

Some limitations:
- It is not possible to automatically and robustly deal with
faces with more than 4 edges, even if we could choose 4 corners,
as it would create T-junctions and then the automatically propagated
constraints may become inconsistent (limit cycles). It is still
possible to deal with a subset of these cases, but it is lot of work,
much more than this commit.
- In the quadrilateral faces, the angle corners are computed from the
curve tangents but do not use the face normal, so large concave corners
are not rejected with the current approach.
- Currently, the points are uniformly distributed on the transfinite
curves. It should be possible to detect existing transfinite curves
with non-uniform distributions and propagate them.

fix gmsh.option.setString("Mesh.MshFileVersion", "x.y") -> gmsh.option.setNumber("Mesh.MshFileVersion", x) (cf. #914)

explain list[] and #list[] in t2.geo + guard fltk calls in python api with -nopopup so we can test python tutorials