Extending partition selection strategies wrappers

[dvadym]

It contains the following new properties/methods:

1. Exposing epsilon, delta, max_partition_contributed properties
2. probability_of_keep method, for exact probability of keeping partition
   For laplace/gaussian strategies:
3. noised_value_if_should_keep which returns (count+noise) if count+noise >= threshold
4. threshold which is used for selecting partitions