Investigate algebras for measuring data leakage

## Current state
Ibis models leakage as binary leak/no-leak

## Future work
We should use an algebra to model how much leakage there is of each different type of data. Possible approaches include [differential privacy](https://en.wikipedia.org/wiki/Differential_privacy), [LDP](https://en.wikipedia.org/wiki/Local_differential_privacy), [k-anonymity](https://en.wikipedia.org/wiki/K-anonymity).