refactored materials.xml and updated poisson-dd-np problem accordingly

each model has its own parameters, and each model may have specific models
-> DD -> lattice_scattering ...

therefore the materials.xml file has now a different markup to reflect that
-> introduced new viennamaterials 'model' accessor, and used it in poisson_dd-np
for accessing the lattice_scattering parameters for the dd model, looks like:

          NumericType alpha_n_value    = device_.material_library()->query_value(
            vmat::make_query(vmat::make_entry(device_.matlib_material() , material),
                             vmat::make_entry(device_.matlib_model(),     material::drift_diffusion()),
                             vmat::make_entry(device_.matlib_model(),     material::lattice_scattering()),
                             vmat::make_entry(device_.matlib_parameter(), material::alpha_n()),
                             vmat::make_entry(device_.matlib_data()     , material::value()))
          );

-> it is not super convenient, but it is flexible .. later on we can build a convenience layer on top of it
quicksafe: in the middle of implementing 'ionized impurtiy' scattering