added 'set_name' and 'scale_values' method to 'quantity'

allows to conveniently copy a quantity holding, for instance,
the electron density and scale its values from m-3 to cm-3,
user code:

QuantityType & electron_density_cm = problem_description.add_quantity(electron_density);
electron_density_cm.set_name(viennamini::id::electron_density()+"_cm");
electron_density_cm.scale_values(1.0E-6);