Merge branch 'feature/use_is_convertible_instead_of_conversion' into 'master'

[cleanup] replace last usage of Dune::Conversion

The deprecated type trait Dune::Conversion was still used in dune/common/densematrix.hh and will be replaced by std::is_convertible in this merge request.

This is a follow-up to merge request !37.

See merge request !45