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

Handle the case of base_link with child links connect with fixed joints
and no root joint provided.

Handle future release of Eigen3 

With next version of Eigen3, it seems that there exists a missing
initialiser when calling the cast operator.

With newer Eigen 3 version, the incomplete namespace specification
leads to compilation errors.

Eigen::ProcuctReturnType becomes Eigen::Product in Eigen version >=
3.3.0.