associated factory.

  * Add dependency to hpp-geometry
  * Define new CapsuleBody class that derives from Body.
  * We assume that the body has only one inner object of type
    hpp::geometry::collision::Segment that represents a capsule.
  * Beside a CkcdAnalysis vector attribute, class has a segment pair
    attribute to store self-collision pairs.
  * When adding inner and outer segments, self-collision pairs are
    added.
  * When computing distance for a distance pair, kcd is used for
    segment-polyhedron analyses, and a fast distance computation
    is used for segment-segment pairs.
  * Define method to compute minimum distance for all pairs of a body.