Divide the speed vectors into 2 sets: slow and fast speeds

[felipe-m]

These vectors are the halfstep (HS) periods and half microstep periods HuS
For fast speed (when slewing) we don't need the halfstep periods (so take them out)
For slow speeds, all the HuS are the same, so there is no need to have a vector for all of them, just one value.
Create a function to get the periods from the given speed.
Instead of having the vector as floats of microseconds, it would be better to have them as integers (maybe shorts), because the timers of the interrupts only accepts integers microseconds.