This reverts commit 1e9a2e831336ca7de622b3ea91eb8d0c4706d296.

Conflicts:
	dune/stuff/la/container/eigen.hh

Conflicts:
	dune/stuff/la/container/common.hh

* this way one can call as_imp() when matching against an interface

* not sure if gcc and icc understand this

so we get the have_module macro for it

* to avoid clashes with Dune::Exception