fixed Prestress

src/elasticity/assemble/IElasticity.hpp

@@ -76,6 +76,7 @@ public:
 
   double Residual(const Vector &u, Vector &r) const override {
     r = *Prestress;
+    r = 0;
     for (cell ce = u.cells(); ce != u.cells_end(); ++ce)
       Residual(ce, u, r);
     r.ClearDirichletValues();