Add MATLAB support for std::optional

[ProfFan]

Summary

• map std::nullopt to MATLAB [], and MATLAB [] back to std::nullopt
• recursively use the contained type's normal MATLAB conversion for engaged optionals
• preserve shared ownership for wrapped objects by copying optional class values into proxy-owned shared_ptrs
• represent std::optional<std::pair<T, U>> with the wrapper's existing two-output convention, returning [] for both outputs when disengaged
• document the MATLAB optional contract and add generator coverage for values, objects, arguments, properties, and pairs

Motivation

The existing generator dereferenced optional results but still instantiated a shared_ptr<std::optional<T>>. That produced invalid generated C++ and provided no representation for a disengaged optional.

Validation

• python3 -m unittest discover -s tests — 112 tests passed
• generated the complete GTSAM MATLAB wrapper, including 406 optional conversion sites
• compiled the full 6.6 MB generated wrapper with Octave 11.3 MEX headers
• passed Octave runtime smoke tests for:
  • disengaged and engaged std::optional<Pose3> returns
  • MATLAB [] passed to an optional argument in ISAM2.update
  • optional vector and matrix values
  • std::optional<std::pair<Vector, Matrix>> outputs