Rework center of mass algo + Python API

Use the name for all the algo related to centerOfMass computations. All
of them now return the center of mass position of the whole rigid body
system. Add also to compute only the position and velocity of the CoM

Add missing namespace

Rework SRDF parsing + correct GeometryModel parsing from URDF

The geometries are now loaded by reading again the URDF file. From now,
one has to build a model first from a URDF file, then to load the
geometries according to this model.

This reference is mandatory, just because a GeometryModel is linked to
a kinematic Model

Handle SRDF file parsing for activation of collision pairs

This new feature allows to activate collision pairs according to an
SRDF file

Enhance documentation + correct bug in Frame + minor modif

There were some bugs in the algo + rename framesForwardKinematic in
framesForwardKinematics (follow the API)