Some variable names are unclear

[rokusottervanger]

There is a number of variables that represent dimensions and quantities that are referenced in the explanation of the control strategy in the README. Naming of these variables is not consistent, though. Some variables are just unclear in what they represent from their names. This holds for (but is not limited to):

• l: the lookahead distance (not mentioned as such in the README yet)
• current_tf
• some of the controller state members, such as current_x_vel (which is the setpoint, I suppose)

[Timple]

Some variable names are also present in the pictures in the README.md.

I propose to name them according to the pictures.