GModel : getElementByCoord for a given dim (this fix dgFunctionevaluator when the point is on the boundary)

MElement : correct bug in 1d jacobian (please check this commit is correct, I'm surprise such a bug survived so long unnoticed)

Reverted all fixes for Post/PViewDataGModelIO.cpp, I was just generating
inconsistent post-processing data, and geuzaine's fix on fix created
an offset between vertix indices saved by GModel::WriteMSH and
PViewDataGModel::writeMSH. 

AND a hotfix to Post/PViewDataGModelIO.cpp which was saving NodeData with
negative MVertex indices in .msh files.

gluLookAt() ) and tou be able to turn on stereo device

GmshSetOption("General", "Stereo", 1.);
GmshSetOption("General", "Camera", 1.); 

Camera : to use camera instead of rotation matrix 

Stereo : to turn on/off the stereo rendering (for device with left and
right views in the two buffers (left and right) of OpenGL : Works in utils/api_demos/mainGlut

utils/api_demos/mainGlut  :

- introduction of a class Camera to use function glulookat() instead of
rotation matrix in openGL viewport.

- A special implementation of the Trackball for stereo mode to be
consistent with other code (Abaqus, Catia, ...)

- adaptation of Drawcontext in Camera mode (works for mainGlut but to
  be validated in gmsh

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memory leak)

sparsityPattern

times

(10 times faster for a typical CG matrix on p6 tet)

used in linearSystemPETSc for now)

this on platforms without full opengl... on ipad? :-)