Add class member to track initialized Points
Remove vector method and redesign flying principle

Now the plan is that when the drone flies somewhere, it first turns
and then traverses the horizontal and vertical plane in the direction of its
destination.

This avoids the need for complicated calculus equations.

Also, the documentation is modified to include data types.
Add navigation method boilerplates to Point
Bugfix

In Point.distance(), a coordinates with the value of 0 were being ignored
because (0 == False) == True.

Fix: Check if the number is None or not rather than if it is True.
Finish Point.heading()

The implementation is a bit complicated and I find it somewhat
redundant... I'm open for any ideas as to how to improve it.

Especially the first two blocks, in which I look for the X- and then the
Y-axis to see if the points are parallel to each other and if the point
exists in both dimensions, are repetitive... And yet abstracting the
code to make it DRY would have led to a whole bunch more code. So I left
it that way for now.
Finish Point.slope()
Finish Point.slope()

I had a tough time with this, I kept on returning the negative slope
angle, even though I didn't need it - the value was negative to begin
with.
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.
Merge branch 'point_methods'

Conflicts:
	apiselectronica/apis_electronica.py