Redesign create_materials

In input/src/include/input/userinput.h:

• For each material appearing in solver/initialize_material.h, create a corresponding material property struct holding userinput values (e.g. KimFccProperties for KimFcc). Leave them empty initially, move values from Userinput into such structs later. Note: each field of such a struct should have a default value (as in userinput).
• A new field material of type std::variant<KimFccProperties, ...> (replace ... by the remaining property structs you created).

In solver/initialize_material.h:

• Rewrite the create_material function, so it creates the correct material appearing in the newly created material field of Userinput. Use std::visit. At this point move everything specific to a certain material from Userinput into the property structs from the above steps (there are already some structs which could be converted into material property structs: MeamPotentialParameters, NNParameters).

In input/src/userinput_json.cc

• Make the material field in Userinput serializable (first need to make the property structs serializable). An equivalent change was recently done in lattice/latticeGenerators/lattice_generator_parameters/src/lattice_parameters_json.h by @stefazim for LatticeGeneratorParameters::method:

  std::variant<SymmetricGrainboundaries<Dimension>, NanoIndentation<Dimension>,
               ReadCoordinates>
      method = NanoIndentation<Dimension>{};

To make a struct serializable, use: NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT as in lattice/latticeGenerators/lattice_generator_parameters/src/lattice_parameters_json.h.

To make a variant serializable do something similar as:

// std::stringview <-> Variant type mapping
template <>
struct utils::json::VariantName<lattice::SymmetricGrainboundaries<3>> {
  static constexpr std::string_view name =
      std::string_view{"SymmetricGrainboundaries"};
};

template <> struct utils::json::VariantName<lattice::ReadCoordinates> {
  static constexpr std::string_view name = std::string_view{"ReadCoordinates"};
};

template <> struct utils::json::VariantName<lattice::NanoIndentation<3>> {
  static constexpr std::string_view name = std::string_view{"NanoIndentation"};
};