Replaced Vertex class by Edge

[remi-san]

As told in issue #39, the chosen name was not the right one.

Here's the right one according to the chosen semantic field (vertices / edges).

[codisart]

I'm not sure that edge is the right term, it imples the object is only a part of a geometry rather a geometry type himself.
I'd rather use Vector as the name of the class and edge in the usage of the Vector class inside the Polygon class.

[remi-san]

Maybe not the right one, indeed, but far better than the misuse of vertex ;)

I'm not a big fan of edge neither: it was just a way to stay in the same semantic field as vertex.

The problem with vector is that it represents an origin (ok, we get from), a direction, and a magnitude : we have a target point instead (::to()), but the behavior of the vector is also present: ::destination()

If we want to stay in the semantic field of geometry, we should prefer the use of edge (or segment) as we want to represent a line between two points who can be a part of a geometry (or not). If we want to represent a point, a direction an a magnitude, we should use vector.

I'm not that sure we want it to be a geometry.

I think the real problem is to ask: how will we use this object? It has a lot of usages which are very distinct. We might want to split it in different classes.

Nonetheless, it's clearly not an edge which is just a point of the boundary of a polygon.