Adjust for configurable security

[omershlo]

Cryptanalysis of class groups is evolving. There should be a simple way for a consumer of the library to configure the required security level of each protocol. Usually this is a risky parameter to expose however in the specific case here it is wise because it might be that some user will want to be more conservative than others or that some new result will require a quick change.

Perhaps a feature is the best way to do it.