message: Change <variant> include to cpp17/variant
diff: #include "eos/morphablemodel/PcaModel.hpp"
  #include "eos/morphablemodel/Blendshape.hpp"
  #include "eos/cpp17/optional.hpp"
  +#include "eos/cpp17/variant.hpp"
  #include "cereal/access.hpp"
  #include "cereal/cereal.hpp"
  #include "cereal/types/array.hpp"
  #include "cereal/types/vector.hpp"
  -#include "cereal/types/variant.hpp"
  #include "eos/cpp17/optional_serialization.hpp"
  +#include "eos/cpp17/variant_serialization.hpp"
  #include "eos/morphablemodel/io/eigen_cerealisation.hpp"
  #include "cereal/archives/binary.hpp"
  #include <vector>
  #include <fstream>
  #include <algorithm>
  -#include <variant>
  namespace eos {
  namespace morphablemodel {
  