Prv-accountant

This release adds PRVs for common privacy mechanisms:
- Gaussian mechanism
- Laplace mechanism
and also PRVs for pure-DP and approximate-DP guarantees.

This release also updates the accountant class to allow heterogeneous composition more easily.

This release adds some checks to improve the reliability. For instance, we check that delta is in a range not limited by floating point arithmetic issues and we make sure that the requested number of compositions does not exceed the initially specified bounds. Furthermore, we improve the robustness of creating the computational domain. Previously, floating point issues sometimes caused the mesh size to deviate slightly from the requested mesh size.

Improve robustness of PoissonSubsampledGaussianMechanism and refactor RDP accountant to reduce number of dependencies.