Constructor of FlatMultiIndex causes problems on clang 3.9

With clang 3.9 this constructor

  template<class... T>
  constexpr FlatMultiIndex(T&&... t) :
    std::array<size_type,1>(t...)
  {}

causes an error when from a PDELab test (testdunefunctionsgfs) an std::vector of these is instantiated (apparently, this constructor matches for tuple size 0). Tentative fix coming soon.