full_value assignment fails when unit present

[rozyczko]

PROBLEM:

• UnitError when accessing Parameter.full_value after interface assignment
• Error message: "Expected unit Å, got dimensionless"
• Occurred when accessing: any parameter in any library after redoing bindings of the interface
• Also affected arithmetic operations between parameters

ROOT CAUSE:

• When model.interface = calculator is assigned, generate_bindings() sets up callbacks
• The callback function returns raw numpy.float64 values (dimensionless)
• Parameter.full_value property tries to compare dimensionless callback value with scipp.Variable that has units
• This comparison: scalar != self._scalar triggered UnitError