* Added query methods by participation, i.e. List findTasksByParticipation(String participation, UserRef user);
* Use IdentityRef (UserRef, GroupRef) where appropriate, i.e. void assignTask(long taskDbid, UserRef user);
* changed Participant to Participation, i.e. Participation.getType() = 'candidate'
It looks a little cumbersome in the beginning, but the distinction between user and group references was necessary. I.e. assignTask(long taskDbid, UserRef user) now makes clear that task can only be assigned to users, not groups. The previous signature 'void assignTask(long taskDbid, String id)' didn't reveil that intention.