Add support for parallel composition in dp-accounting

[ryan112358]

In some mechanisms it is helpful to partition the dataset and run a different mechanism on each subset. The privacy properties are then the worst-case across all subsets. It would be good to have support for this in dp-accounting.

[pritkamath]

Thank you for the suggestion.

Supporting parallel composition in full generality is a complex change, requiring significant changes to the current accountant design. In particular, it would require maintaining the PLD or RDP values for each sub-mechanism part of the parallel composition. It gets especially complicated, if one further composes a parallel composed mechanism with another mechanism.

For this reason, we are hesitant to work towards supporting this. If you just want to apply parallel composition at the outermost level, then as you are mentioning, you can simply apply the accountant on each sub mechanism, and take the worst case.