add wall following cflib demo

[knmcguire]

We have had the wall following app for a while in the crazyflie firmware. There used to be a python version made back when the SGBA work was done, but at one point all development was done using only the c version of the wall following algorithm.

I've now ported the c code back to python, so that we can have a cflib example of the wall following demo.

[krichardsson]

To me this is OK to merge, especially since it is a direct re-write of the C-code and you want to keep the 1 to 1 mapping.

There are room for improvements though, some general ideas would be:

1. Use constants for numbers to describe what they might mean
2. Use math.pi instead of 3.14
3. Use math.radians() and math.degrees() when converting between radians and degrees
4. Add a function for cleaning up multiranger values instead of repeating code
5. A more liberal use of variables that describes the meaning of a calculation. There are some cases where calls to logic_is_close_to(), contain a fair amount of code. Readability would improve by first assigning the calculation to a variable with a descriptive name, and secondly using the variable when calling logic_is_close_to()

[knmcguire]

Thanks for the tips! I've implemented a few already but I'll add issues to both the crazyflie-firmware and crazyflie-lib-python to implement these in the future. It would be good to keep the two versions synced up.