Dummy drugs

[legaultmarc]

Sometimes, we know that an individual uses a drug class without knowing the specific molecule.

This needs to be supported when registering drug users (e.g. register_drug_user("user0123", "beta-blocker")). The correct behaviour is also expected when querying the manager (e.g. get_drug_users).

Implementation could be based on ATC codes.

[legaultmarc]

A "dummy" phenotype is also needed to represent complex hierarchies. These would be regular phenotpyes, but with forbidden get_data(). They also wouldn't get listed unless explicitly asked for.

See Issue #14