Ambiguous namespace in basisgridfunction.hh

In functions/basisgridfunction.hh there is a namespace Functions defined. However, if one is using using namespace Dune, what is probably quite common in user code, this may lead to ambiguities since in dune-functions often a namespace Dune::Functions is used. I would therefore suggest to rename the namespace in the fufem code, however, I don't want to break existing user code of others.