Initialization of link travel times

[becarie]

For the first time step assignment, the travel of time of the links must be initialize from an init function of the flow motor and not from the default speed value of the layers.
The MFD speed functions shall calculate the speed value per mode for each reservoir at time 0, based on the number of vehicles in the reservoir at time 0

[floriangc]

The default speed defined by the layer is used to define the initial speed to simulate a pre-charged reservoir, if we use the MFD speed with a 0 accumulation we force the reservoir to be at maximum speed.