Implement weighted combine methods with DP

[LaRiffle]

We can start by asking each time the DP-size of the dataset for the weighted aggregation (and cut a part of the privacy budget accordingly), like for the mean function. In a second step (independent of this issue), we will try to cache the DP-size to avoid spending budget on this).

For tricky methods like variance / covariance, we will need further implementations, so let's put this on a different issue for later.

[LaRiffle]

@Jasopaum I have added the description, let me know if you want to discuss this live!

[LaRiffle]

• [ ] Mean (with DPrivate weights)
• [ ] Sum
• [ ] Min
• [ ] Max All other need further investigation