add named tuple for result of schedule generator and add max_episode_steps to the schedule generator

remove variable observation_space since it is not really used anywhere and adjust examples in doc to match the code

#79 bugfix sphinx generated documentation #161 observation documentation  #159 documentation about new INFO

renamed action_required instead of entering and remove two if False conditions in sparse_rail_generator

#155 return in info field whether agent is malfunctioning; renamed "actionable" to "entering" which describes content better