def add_team(team)
      team = Team.new(team.to_s) unless team.is_a? Team
      raise "a team with a name similar to '#{team.name}' already exists" if self.get_team(team.name)
      @teams << team
      team
    end
def add_team(team)
      team = Team.new(team.to_s) unless team.is_a? Team
      raise "a team with a name similar to '#{team.name}' already exists" if self.get_team(team.name)
      @teams << team
      team
    end
Add a new team. The argument is either a team (possibly already with members) or the name of a new team.
 The team's name must be unique in the tournament. Returns the the team instance.