Adds in dgplate : 1) Non linear solver (Newton-Raphson) 2) cohesive law

(fracture in bending OK) 3) lua version of code 4) a test case with 3rd
order quadrilateral element. 
Some modifications are made in linearSystem (add a function to compute
norm of RightHandSide) and in dofManager (add a function to have the
fixed dof outside dofManager)