ENH: change default init in volumetricMesh reader

A surfaceMesh will not be automatically extracted when
reading a volumetricMesh due to the long time necessary
for that process, which will be improved later on.
(vtkDatasetSurfaceFilter is 100x faster)

The user will have to call computeAttachedSurfaceMesh to
extract the surface mesh from its volumetricMesh if he
needs it.