* Introduce AlwaysFalse, a helper template for dune_static_assert().

* Turn STRIP_CODE_COMMENTS on in Doxyfile.in.  Otherwise, doxygen seems to
  remove comments from \code...\endcode blocks.  This has the side effect that
  comments are no longer stripped from the source code doxygen presents via
  the web interface, which is probably a good thing in itself.

[[Imported from SVN: r5616]]