Introduction of a clutch slipping speed depending on gradient to ensure
that a powertrain with low torque can start uphill if it is possible by
increasing ICE speed during slipping:
- fixes the following issues :
    - #331
    - #367
    - #372, #373, #374, #375
    - #393, #439, #446, #449, #478, #481, #488, #494
    - #420
    - #573, #582
- computation of ice torque necessary to perform a given acceleration
for current gradient and slope based on built-in functions
- adding a function to compute the interpolated min speed necessary to
get target torque
- additional correction of target torque (+1%) as safety for overloaded
vehicle to make sure worst case are dealt with (#393 and #420)
- also tested on generic vehicles not concerned by the issue, with big
ice / high torque (bus and tractors) with zero or very limited change in
 consumption in those cases (mean consumption variation for vehicle
presenting one : -0.02%)