The current behavior of Gmsh when drawing 3D scalar fields is not coherent
with what is done in 2D: to draw filled iso-values or continuous maps in 3D,
we should ideally do volume rendering. Right now we draw the iso-values
(i.e., surfaces), which is both not coherent with the definition of
continuous maps or filled iso-values, but is also annoying when trying to
visualize fields that are constant per element (since in that case nothing is
displayed!)

Until we can do volume rendering, I've changed the behavior to draw the
solution on the boundary of the 3D elements. It's rather slow, but it's the
right thing to do visually.