[patch] Acceleration Graph in Convoi Info Dialog
This is the 1st promissed patch from simutrans experimental: the acceleration graph.
It shows the convoy speed starting from 0 km/h in steps of 15 seconds according to the current weight, friction given by slope and curve of the vehicles, etc.
The graph use exactly the same code, that actually accelerates the convoy. Therefore I had to divorce the calculation part from the setting part in simconvoi_t.calc_acceleration().