[Python] Updating Python tests to corresponds to the API 

[Python] Updating Python tests to corresponds to the API ( e.g we add frames for every link and joint we encounter in urdf )

Topic/difference interpolate fix

[C++] The method forwardKinematics now selects by itself the section of the right DoFs over a full vector

Append cflags to the .pc file 

Topic/visuals without fcl

Add missing forward declaration.

Replace nbody to njoint and activate all collision pairs by default

Make coverage and cppcheck optional and dependant of the current branch

[C++][CMake] Merged fake-fcl with fcl.hpp (splitted in .hpp and .hxx file). Propagated flag -DWITH_HPP_FCL to all binaries

[C++][Python] One can contruct a GeometryModel and Data even if hpp-fcl is not found. All stuff related to collisions is not available but geometryPlacements are ( for visualisation in the viewer especially).

[Python] Add spatial copy constructor