Reset the CP model by default

Added an argument to Scheduler constructor to reset the CP model of the nodes (true by default).

When creating a new Scheduler, it is counter-intuitive to the users that the CP model is not reset, as a previous scheduling on some nodes (for example before graph transformation) will lead to unexpected and hard to understand scheduling behavior @axelfarr @cmoineau...

@axelfarr With this MR, it will not be required to call reset_scheduling() before new Scheduler creation.