SmartNoise privacy/accuracy fixes

[raprasad]

(Created to reflect PR 315) Addressing several bugs found/fixed by @shoeboxam:

• A sign in the analytic gaussian was not being set correctly, resulting in reduced sigma
• noise scale in the gaussian mechanism should not be squared
• Accuracies for disjoint queries were incorrectly multiplied by the category length
• Accuracies for the gaussian mechanism were incorrectly using values for the analytic gaussian
• Accuracies would not coerce integers to floats when necessary.