FIX :

Related to #897 an #383 

* A child cannot force a check more than once per hard state
     => Add a boolean to disable the check : checked_by_child

* Pep8 style :
     => Redundents \ into ()
     => Line length
     => Empty line with whitespaces
     => Unused import
     => Comparing to False must be done with not 

* Remove an obviously useless line from and condition. : 
    => Useless to test if we are SOFT after testing SOFT and not PENDING
    => Useless to set to SOFT something that is already SOFT