Update tdd_example

[oyvindeide]

This adds a new example for the tdd workshop,

corresponding issue text:

You are an intrepid explorer, navigating a scenery of rolling hills at night. It is completely dark, and your only source of information is your handheld computer which will give you the next point on your journey. You have heard that there exists a treasure here, and you only clue is that the treasure is located in an incline where the slope between your current position and the next coordinate is above X. Your task is to navigate this landscape and locate the coordinates of the treasure.Example:
Given a list of coordinates (your test input) (x, y):
[(1, 1), (2.0, 1.8), (2.5, 2.1), (3, 2), (4.3, 7.2), (5.3, 6.2)]
we can calculate the slope along the coordinates:
[0.8, 0.6, -0.2, 4.0, -1.0]

given a threshold of 1, the coordinate we are looking for is: (4.3, 7.2)

[hnformentin]

Very good contribution!
I made some comments for consideration, if you can address them that would be awesome :-)

[lars-petter-hauge]

Nicely done! I had some comments/suggestions as well, it's always easy to create those :D

[lars-petter-hauge]

Nice, thumbs up from me, just remember to squash the review commits before merging ;)