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.