[Feature Request] Security guard: percentage of elements that passes the threshold

[bartvanb]

Problem description
The contingency table should not reveal any information and should thus only be used for categorical values that contain sufficient values that are the same. However, it is very difficult to distinguish beforehand what is a categorical value and what isn't. At the moment, the security guards at the disposal of the node admin are:

• setting a privacy threshold N. All values below N are replaced by 0-<N-1>. Also, the table is not shared at all if no values >=N are present (to prevent that unique values are shared)
• Setting which columns are allowed to be requested (by setting whitelist/blacklist)

Desired solution
An additional security guard would be to require a certain percentage of elements >=N. E.g. if set to 50%, at least half of the elements of the contingency table should be >=N, or otherwise an error would be raised.
This guards should be an environment variable set by the node admin, with a default (10%?) set inside the algorithm image.