Geometry volumes

Feature request

What does this feature / why does DuMux need it:

When building dune geometries and calling geometry.volume() this only yields a rough estimate of the real volume, at least for non-tetrahedral geometries. This is also clearly stated for example when looking at MultiLinearGeometry class:

    /** \brief obtain the volume of the mapping's image
     *
     *  \note The current implementation just returns
     *  \code
     *  integrationElement( refElement().position( 0, 0 ) ) * refElement().volume()
     *  \endcode
     *  which is wrong for n-linear surface maps and other nonlinear maps.
     */
    Volume volume () const
    {
      return integrationElement( refElement().position( 0, 0 ) ) * refElement().volume();
    }

I think we should therefore check our code and see where this is used and replace it, if possible, with Dumux::volume(...). At least I forgot that recently and used it to calculate barycenters, which then gave me wrong results (leading to non-convergence).

The same also holds for faces in 3D.

Some options to discuss:

We could do the following

• Fix volume function and check if geometry if affine
• Pass volume / center / normal to contructor of scvs/scvfs which we get from geometryHelper. I think this is anyways better. (We partly do this already for other CVFE schemes)
• We can then easily exchange the geometry helper to be more exact...
• Provide integrate and average functions over geometries that prevent inconsistencies. Main source of error is by for example integrating exactly but then dividing by wrong volume.