Change smoothly the cumulated_error when error sign change

DO NOT INSTALL IF YOU ARE NOT INVITED TO DO SO

This release change the behavior of the cumulated_error in the PI algorithm.
When the error sign change (ie the heating conditions have change), the cumulated_error is divided by a factor (2) so that the transition will be quicker.

The cumulated_error is change resetted when the target temperature change.

This should fix:

1. @ikoz issue: when heating is now needed and the VTherm have accumulated a lot of error, the transition is slow,
2. @maia issue: when target temp change but heating conditions are the same (should not heat in this case), the reset of error_accumulation make the heater starts heating.