slurmdbd: Implement Association API

[tazend]

Classes

• Association class providing access to all possible members of slurmdb_assoc_rec_t
• Associations class, acting as a custom collection (dict) that holds all the Association instances.
• AssociationFilter class acting as a wrapper around slurmdb_assoc_cond_t to search for associations matching specific conditions (server-side SQL filter).

General Functionality for the Associations class

• modify
• load
• delete
• create

Other

• Documentation
• Write tests
• Write examples