Cosmetic change

  Fix documentation.
Add a boolean parameter to method Joint::addChildJoint

  - the parameter specifies whether computation of the joint position in
    parent frame is required or not.
Modify operator<< in Device and Joint so that output defines a dot graph

  - copy paster corresponding part from log file to graph.dot file and run
    dot -o graph.pdf -Tpdf graph.dot
Fix some bugs in computation of center of mass and Jacobian

  remove division by zero when mass of a sub-chain is zero,
  set COM Jacobian to zero when resizing.
Merge branch 'master' of https://github.com/florent-lamiraux/hpp-model