Implement path following controller

[ufechner7]

Precondition: The trajectory is defined by:

1. a function that has as input an angle and as output azimuth and elevation
2. in a second step, the tether length and and two expension parameters (0..1), one for azimuth and one for elevation can be added as inputs

Todo:

• implement the function, defined above in Julia
• implement a controller that creates an attractor point based on this function and of the current position of the kite
• implement an outer loop that minimizes the error

[ufechner7]

Good paper on path-following control: https://www.mdpi.com/1996-1073/15/4/1390