scaling_factors accessor

[nollety]

Provide a scaling_factors accessor to compute the scaling factor to use in order to reach a target amount along the molecule dimension.

The accessor would be useful in coordinate with the rescale accessor:

ds = joseki.make(identifier="afgl_1986-midlatitude_summer")
factors = ds.joseki.scaling_factors(
    {
        "H2O": 40 * ureg.kg / ureg.m**2
    }
)  # returns {"H2O": 1.3403081516598105}
ds.joseki.rescale(factors=factors)