Return list of potential obstacles in Point.obstacle_between()

This still isn't finished. At the end I don't want potential obstacles,
I want real obstacles. But this will be the last commit on this branch,
I'm merging into master and rebranching. The reasons:

* I don't want to halt work on master
* The point methods are advanced enough to take a look at if you're
programming flight patterns
* I need the exception that's already on master

I think that in the end a good solution could be to return the points
that are in the way and put the exception in Drone(). It can then be
handled by the drone and the drone knows where to fly.