Implement clutch slipping speed for vehicle with small ICE

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, #509, #521, #523, #534, #543, #569, #630
  • #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%)