feat: Add stream insertion operator and unit symbols for various quan…

[VPRamon]

This pull request introduces a streamlined and user-friendly way to print quantities with their unit symbols, making output more readable and intuitive. The core changes revolve around adding a stream insertion (operator<<) for quantities and ensuring every unit type provides a symbol for display. The demo and code generation are updated to leverage this new feature.

Key changes include:

Printing and Display Enhancements

• Added a generic operator<< for Quantity types in ffi_core.hpp, allowing quantities to be printed directly with their unit symbols (e.g., 1500 m, 42.5 km). This significantly simplifies and improves output formatting.
• Updated the demo (demo.cpp) to use the new operator<< for all quantity outputs, replacing manual .value() and string concatenations. [1] [2]

Unit Trait Improvements

• Added a static symbol() method to all UnitTraits specializations for angular, length, and mass units, providing a string representation for each unit (e.g., "m", "kg", "°"). [1] [2] [3]

Code Generation

• Updated code generation in gen_cpp_units.py to automatically emit the symbol() method for each unit trait, ensuring consistency and reducing manual errors.

Core Library Includes

• Added necessary includes (<iostream>, <string_view>) in ffi_core.hpp to support the new stream insertion and symbol functionality.…tities