Fixes problem in exportOperatorMatrix demo app
* Elimination of DoFs associated with Dirichlet boundary conditions was not working correctly. Fixed this by inserting a call to set the primitive flags correctly via setMeshBoundaryFlagsOnBoundary() * The exportOperator() function now allows to ask for unsymmetric elimination, which we can use from within the app. * App is now more robust and will detect wrong operator names on cli.