python interface: all std::vector<> in element constructors are declared as const, allows easy python binding

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(useful for simulating synthetic foam-like materials)

Lift And Drag better, but dgFunctionEvaluator returns some out of range values (bbox ?, precision ?)

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Fracture can be prescribed by a different law on interface
Explicit system is implemented with petsc vector operations

Modified FunctionEvaluator And MeshMetricBasedOnHessian that now take groups instead of solution as arguments

Adds an option to switch elementary and physical tags in loaded meshes, and fixes a bug with geo-mesh matching.

better quad meshing

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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)