message: Use cpp17/optional_serialization.hpp header
diff: #include "cereal/cereal.hpp"
  #include "cereal/types/array.hpp"
  #include "cereal/types/vector.hpp"
  -#include "cereal/types/optional.hpp"
  #include "cereal/types/variant.hpp"
  +#include "eos/cpp17/optional_serialization.hpp"
  #include "eos/morphablemodel/io/eigen_cerealisation.hpp"
  #include "cereal/archives/binary.hpp"
  