[!240] Allow number types as entries of matrix and vector types

Merge branch 'allow-blockvector-of-doubles' into 'master'

ref:core/dune-istl So, for example, you can write

    BlockVector<double>

instead of

    BlockVector<FieldVector<double,1> >

and it will do what you expect it to do. With this patch, any type for which
Dune::IsNumber<T>::value returns true will end the recursion.

See merge request [!240]

  [!240]: gitlab.dune-project.org/core/dune-istl/merge_requests/240