Introduce AdvectionField::scalar_extractor

[bangerth]

#661 has code like this, that I suspect is repeated in other places as well:

     const FEValuesExtractors::Scalar solution_field
       = (advection_field.is_temperature()
          ?
          introspection.extractors.temperature
          :
          introspection.extractors.compositional_fields[advection_field.compositional_variable]
         );

The AdvectionField class already has a number of convenience functions that together with Introspection yield components, blocks, etc, that correspond to a particular advection field. We should also add such a function that creates an FEValues extractor to make code such as the one above simpler.

[yinghe616]

so I think this issue is OK to close?

[bangerth]

Yes. Thank you!