Custom Operator Registry Support

[ajay-mk]

Custom Operator Registry Support

This PR refactors Operator logic from depending on a predefined OpType enum to a runtime OpRegistry, enabling users to define custom operators beyond the predefined set.

Major Changes

• OpRegistry: a registry for operator labels and their classifications
• mbpt::Context now holds an OpRegistry along with the CSV setting.
• There are two predefined registries available (minimal and legacy).
• Some labels are reserved for internal use (antisymmetrizer, symmetrizer, kroneker, overlap), and cannot be registered.
• OpType enum is removed, all related logic is gone:
  • OpMaker constructors now use strings.
  • OpConnections now uses strings to specify operator pairs. For example: {{L"f", L"A"}, {L"g", L"A"}, ...}
• Built-in operators (both tensor and Operator level) check the registry if operators are registered.
• Perturbation-related operators now support higher-order perturbations (up to 9).
• mbpt::Context manipulations are thread-safe, and can be toggled by SEQUANT_CONTEXT_MANIPULATION_THREADSAFE option; follows the same pattern as core::Context.

Example Usage

  auto custom_registry = std::make_shared<OpRegistry>();

  custom_registry->add(L"f", OpClass::gen)
      .add(L"g", OpClass::gen)
      .add(L"t", OpClass::ex)
      .add(L"λ", OpClass::deex)
      .add(L"X", OpClass::ex);  // Custom excitation operator

  // set MBPT context
  set_default_mbpt_context({.op_registry_ptr = custom_registry});

  // Built-in operators work as before
  auto t2 = T_(2);  // Uses 't' from registry

  // new custom Operator using OpMaker
  auto X2_tensor = OpMaker<Statistics::FermiDirac>(L"X", 2)();

  // new custom Operator using mbpt::Operator
  auto X2_op = ex<op_t>(
      []() -> std::wstring_view { return L"X"; },
      []() -> ExprPtr { return OpMaker<Statistics::FermiDirac>(L"X", 2)(); },
      [](qns_t& qns) { qns += excitation_type_qns(2); });

  auto expr1 = H_(2) * X2_op * t2;

TODO

• Remove all use cases of L"A" and L"S" and replace them with functions.
• Add documentation
• Cleanup