drawing vertices per element

Reverted patch from bruno until we verify that it does not affect other parts of the code

Right now d in meanPlane we supposed that d corresponds to the rhs of the plane equ:

ax+by+cz=d

This is not consistent with what we do elsewhere, but we should check all the tests
before changing the convention 

(Bruno's commit broke e.g. the Plane Surface test in GFace)

Compute radius of the inscribed circle within an element. Done for Mquadrangle, MTriangle, Mtetrahedron

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Added distance computation geometrical and pde based with -distance command line option. Christophe, we should discuss if we move this to plugin or elsewhere.  

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an example (mesh2mesh.lua) has been added

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