Fix coba, chord algorithm, doc improvement

    - Chord algorithm for (naive) plane recognition and width computation.
    - New organization for computing primitives. Introduction of the concept
      of PrimitiveComputer and specialization. COBA algorithm and Chord algorithm
      are now models of AdditivePrimitiveComputer.
    - Introduction of the primitive ParallelStrip, computed by COBA and Chord algorithms
    - New documentation for planarity decision, plane recognition and width computation.
      Quantitative and qualitative evaluation of COBA and Chord algorithm.
    - Bug fix in COBA algorithm when extending an empty computer with a group of points.