fix: add SMT downshift condition - DroppedSpd>DisengSpd

add SMT downshift condition - DroppedSpd>DisengSpd

Add a condition to avoid downshifting while reaching the disengage speed. SMT long gearshift time would result in difficulties to reach target distance due to torque interruption.

solves issues : #495, #642 and #739 tests performed : generic files + all current art10 issues, results on FC :

• max variation +0.75% on one interurban 32d LowLoad on a vehicle previously not working in 4.2.5
• min variation -0.33% on one UrbanDelivery LowLoad on a vehicle previously not working in 4.2.5
• mean variation observed on all vehicles +0.03%

closes #495, #642, #739