Fixed: Atom::is_ground() didn't check arguments.
Added unit test for Clause::Rel().

Are non-ground literals treated correctly in Rel() and Subsumes()?
Clause::Rel(), Clause::Subsume() look correct for non-ground literals.

Therefore renamed GroundClause to SimpleClause.
Updating a unifier now doesn't fail if the pair was present before.

That wasn't a known bug, but it might have been an unknown one.
Added comparison operator to Clause.