(WIP) Support to PLD composition #259

[ricardocarvalhods]

Description

Solving "Advance composition in PipelineDP" #259

The approach I'm using is:

• Propagates noise_standard_deviation (in the case of NaiveAccountant, it is None throughout)
• When it reaches apply_laplace_mechanism/apply_gaussian_mechanism, if noise_standard_deviation is NOT None, we use it for noise (ignoring eps/delta)
• Otherwise (if noise_standard_deviation is None), eps/delta are used to add noise.

Tasks:

• [x] Add noise_standard_deviation to scalar computations and parameters
• [x] Ajust tests to pass after changes
• [ ] Create tests for scalar computations with noise_standard_deviation
• [ ] Same as above, just to vector sum computation

Additionally:

• I've also did some minor changes, like two TODOs in the dp_computations.py and formatting.
• I see that request_budget receives noise_standard_deviation, do you know why? not sure if it is needed, as noise_standard_deviation is only calculated in compute_budgets.