Renamed JointXXAccessor to JointXX + renamed traits<D>::JointData to traits<D>::JointDataDerived, same for Model.

Conflicts:

	CMakeLists.txt
	src/algorithm/center-of-mass.hxx
	src/multibody/model.hpp
	unittest/CMakeLists.txt

Conflicts:

	CMakeLists.txt

[C++] The stack of joints model & data is now of type joint accessor ( i.e the derived types are encapsulated in a generic joint accessor

Conflicts:

	src/multibody/model.hpp

[Minor] Update name in Data of oMof to oMf + in frames algo get directly the value of the parent joint when the relative placement of the frame wrt parent joint is Identity

Move frame to multibody + add updateKinematics option in com jacobian algo

Add python model parsing from #194

As it is done in src/multibody/parser/python.cpp, one can create a shared_ptr to a new Model in c++,
send this into a python namespace, update it in python, and continue to work on the Model in c++.

An example to use this as a simple parser is given in unittest/python.cpp and models/simple_model.py

(squashed version)

Fix bug when parsing URDF tree + minor clean