Add scale factor for noise

[sebastian]

In our previous Diffix systems we had scale factors for the noise we used. Specifically we would scale down the top group average used for making noisy sums and counts: https://github.com/Aircloak/aircloak/blob/master/cloak/lib/cloak/query/anonymizer.ex#L374-L377

• Validate what scale parameters we should have and whether they still make sense