Add unaligned prismatic joint + minor modifications

Fix bug when parsing urdf file model by considering free floating joint

Still cleaning the parser + add tool

[C++] Raise exception when inertia information is missing when build tree with a specific root joint

Functions defined in headers urdf.hpp and jacobian.hpp are set to inline.

The functions should be inlined, otherwise it breaks with the headers
are used in multiple robots mode.

Set default initialization of variables + expose spatial quantities methods + correct bug

[C++] Build sample humanoid model with joint limits.

  * Velocity and torque limits are choosen randomly from the [0;1]
    interval for 1D joints.
  * Position limits are choosen randomly from the [-1;1] interval for
    1D joints.
  * Torque limits set to zero(6) for freeflyer joint.
  * Velocity limits are choosen randomly from the [0, 1e3]^6
    interval for freeflyer joint.
  * Position limits are choosen randomly from the [-1e3, 1e3]^7
    interval for freeflyer joint.

[Python] Add StdVec::index binding to emulate python list behavior.

  * It is defined only for string vectors for now.

Fix URDF inertia parser

  * Inertial-less links under fixed joints exist for semantic
    purposes, i.e. to provide referential frames.
  * Keep storing these links as fixed bodies.
  * The case of pairs with non-fixed joints and inertial-less links
    still needs to be properly addresses, as the current behavior
    (identity inertia) is clearly bad.

Fix some bugs + update Mac compatibility

May need some change according to the current version of the
architecture