[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

Set minimal version of EigenPy to 1.3.0

Fix licence headers.

Correct bug in geometry parsing

Correct two bugs related to recent PR #76 and #77

PR with minor adds

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

The gain is about 10% for NLE and RNEA.

Minor clean of the framework

Add computation of potential energy

Add computation of Kinetic Energy