Unnecessary or undocumented D.T_predicate

[neo-alex]

🐛 Bug

D.T_predicate was added to the list of user-defined types to support a very specific case (related to multi-agent domains, to enable direct alignment with another existing API). Currently it is poorly documented, so it is not clear that it only relates to the return type of the is_terminal method.

Expected behavior

The main suggestion here would be to (try and) remove D.T_predicate which seems a bit overkill for its purpose (or at least preconfigure it so that it is automatically set to the expected value in single & multi agent cases). There could be interesting alternatives to serve the same purpose without it, e.g. adding a is_agent_terminal(agent_id) method to multi-agent domains and keeping a boolean is_terminal method to know if the whole environment is done.

In case the suggestion above is rejected, D.T_predicate should at least be better documented for the end-user to know how to use it.