egemenimre/ccsds-ndm: Quantity and Type Validation Support

Added type validation for wrapper-typed fields (model_validate): assigning a plain number, Decimal, or string to a physical-quantity field now raises TypeError with a clear message. Added pint and astropy Quantity support (model_quantity): The pint/astropy Quantity objects can be assigned directly to wrapper-typed fields and are automatically wrapped to the correct NDM type. The unit must match one of the accepted CCSDS unit strings for the field; a clear error lists the accepted options if it does not. Added .q() method on all wrapper types (LengthType, DvType, etc.) to extract the value as a pint or astropy Quantity. The backend is selected globally via set_quantity_mode(QuantityMode.PINT) or set_quantity_mode(QuantityMode.ASTROPY).