[C++][Frame] Moved the generation of random string in a new file. Modified the default constructor of frame to produce unique frame (as long as the random strings generated are unique)

[C++][Python] Added missing setters of Frames exposed in python, addFrame now returns a bool depending on the succes of the insertion of the new frame in the operational_frames vector

Conflicts:

	CMakeLists.txt
	src/python/algorithms.hpp

[C++][Python][parser] Added operational_frames in Model as a vector of Frame {name, parent id, placementWrtParentJoint} and exposed it in python. When parsing a fixed joint, a frame is added to the model with the name of the fixed joint

[C++][Python][parser] Added extra_frames in Model as a vector of Frame {name, parent id, placementWrtParentJoint} and exposed it in python. When parsing a fixed joint, a frame is added to the model with the name of the fixed joint

The idea is to separate the real spatial acceleration of a body to the
spatial acceleration containing the gravity field, which is useful in
the context of dynamical equations

[cmake][C++][bug fix] Moved initialization of static const variable gravity 981 from model.hpp to model.cpp. libpinocchio.so is no more empty. Caution : Pinocchio is more a header-only library. One should alwais link against shared library

  * se3::Model::getBodyId was used to retrieve joint id in parsers.
  * Implement new methods to retrieve joint ids.
  * Fix body methods.

  * Add inline specifier where needed.
  * This is done to avoid multiple definition when using the pinocchio
    library as a dependency.

[Python] Binding the different joint models. The exposure is done through the variant. In joints-models, you can specialise the binding for any jointModel you want.

[C++] Add com information in data + compute position, velocity and acceleration of CoM in the same loop + add related unittest

[Major][C++] Added Joint limits to joint model and to Pinocchio Data. Also added an algorithm to compute joint limits. Based on the model of the limits, it computes and store the actual limits in Data. We then have access to the values in Data in python binding. The algorithm is here in future cases where the limits will not only be static but dependent of the configuration

Add attributes in model.hpp
	- names of fixed body
	- indexes of their closer movable parent
	- visuals booleans
	- number of fixed body
modify display method in robot_wrapper
	- compute and send the absolute placement from relative to the closer movable parent

Conflicts:
	src/multibody/model.hpp
	src/multibody/parser/urdf.hpp

[partial cherry-pick from b87a2b53b3]