[WIP][feat] Get neighbors from Vertex map for kpts extraction

• The keypoints extraction is based on laser neighborhood analysis on each point.
• By now, the neighbors are obtained with the points order in each scan line.
• With this MR, a vertex map is built to store the points and to better extract the laser beam neighbors.
• The points which have no such neighbor (left or right) are considered to lay on a depth gap boundary.
• The points which miss some neighbors are excluded from keypoints extraction.
• The line fitting process is changed to avoid computing a PCA on few points (default is 4).
• This new process improves the results on various datasets (motorways, car loop).
• Moreover, we can imagine that the new keypoints on gap boundaries can be really helpful in very features poor environement (on sea for instance).
• These changes are based on the fact that the angle resolution has been computed automatically and a new feature extractor structure exists.
• The combination of this new process with other improvements have to be discussed.

@nicolas.cadart @nick.laurenson PTAL