Support for unit types

I would like to propose a grid that can convert any of the known dune-grids into a grid that "uses" unit safe types. I could propose it here in an MR if you are interested or I can just create a module for it. However, there is a type in the dune-grid interface that can't be aligned with unit safe types: it is the return type of the method volume() which is ctype.

Say that ctype represents the values of \mathbb{R}. Then, all the methods that return values in other dimensions (e.g. local(), center(), etc.) are effectively converted into a tuple of the type of the ctype. As is usual in dune, this concept is covered by the FieldVectors so that \mathbb{R}^d = \mathbb{R}\times ... \times \mathbb{R} = FieldVector<ctype,d>. However, this does not happen with the volume which should be represented as the resulting type of the product of a ctype value d times. In c++, that is using Volume = decltype(declval<ctype>()*...*declval<ctype>());.

There are two ways to fix this:

Asking the implemented geometry to provide the type of the volume as is done for all the other types.

  template< int mydim, int cdim, class GridImp, template< int, int, class > class GeometryImp >
  class Geometry
  {
    // ...
    typedef typename Implementation::Volume Volume;
    // ...
  };

Although 1. is what makes more sense to an interface like dune-grid, I can imagine that it will break many codes downstream. So another possible implementation can be as follow:

  template< int mydim, int cdim, class GridImp, template< int, int, class > class GeometryImp >
  class Geometry
  {
    template<class ctype, int dim>
    struct VolumeTypeHelper;

    template<class ctype>
    struct VolumeTypeHelper<ctype,1>
    {
      using type = ctype;
    };

    template<class ctype, int dim>
    struct VolumeTypeHelper
    {
      using VolumeTypeCodim1 = typename VolumeTypeHelper<ctype,dim-1>::type;
      using type = decltype(std::declval<ctype>()*std::declval<VolumeTypeCodim1>());
    };
    // ...
    typedef typename VolumeTypeHelper<ctype,mydim>::type Volume;
    // ...
  };

Yet a third alternative would be to combine the two options asking whether the implementation provides Volume definition.

Notice that even the jacobians do not assume anything about the underlying type.

So, please let me know if a change like this is possible and desirable. If so, I can push a MR with the corresponding alternative.