Components without density break mixing behavior

[saNistoRoboto]

H2O = Component('H2O',volume=1.0,density=1.0)
Glycerol = Component('Glycerol',mass=1.0,density=1.26) # @todo: had to add density here or glycerol has a volume of none which borks later. Should instantiate with a default value of density.

    target_solution = Mixture([H2O,Glycerol])
    
    print(target_solution)
    print(target_solution.components.items())
    target_solution.volume = 1.0
    target_solution.set_volume_fractions({Glycerol:conc,H2O:1-conc})

If density blank, get error that number of components does not match between specification and real Mixture.

@martintb