Merge branch 'issue/linearalgebra/petsc/Operations.hpp' into 'master'

solve mem. leak in function residuum(...) in petsc/Operations.hpp

See merge request amdis/amdis!92

amdis/linearalgebra/petsc/Operations.hpp

@@ -16,6 +16,8 @@ namespace AMDiS {
 
     PetscReal res;
     VecNorm(r, NORM_2, &res);
+    
+    VecDestroy(&r);
     return res;
   }