Yet another thorough rewrite of big parts of the code:
  * Agents are now defined in terms of their navigate-act-communicate-evaluate(-exit) predicates using user:agent_definition/2.
  * Wherever we used to have four arguments to denote a directed triple, i.e. From-Dir-Link-To, we now have a compound term `dir(From,Dir,Link,To)`. This simplifies a lot of APIs (i.e., less arguments).
  * Simplified the API for creating agents (module DH_AGENT).
  * Vastly simplified the API for testing defined agents (module DH_TEST).
  * DH_TEST_GENERICS has been removed, since we now use RDF_RANDOM_DBPEDIA for random triples and IRIs.