[C++] Make JointComposite part of the JointVariant. Reoganized some include to avoid circular dependencies

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

Conflicts:

	CMakeLists.txt

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

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)

Completed the visitors on JointVariants for each method of the JointBase interface  and moved it to a different file.

[C++][Python] Added algorithm working with joint configurations ( integrate, differentiate, interpolate, random, uniformlySample, distance), their python bindings and unittest

Added missing licenses, corrected compilation bug in python/parsers.hpp. also modified the number of benchmarks to have more relevant results and removed useless lines

[bench][unittest] Reworked the comparison test between pinocchio and hpp : more tests for benchmarks along with functional tests ; removing magic paths to urdf models + modification of romeo urdf model. Also added a specific BUILD_TESTS_WITH_HPP cmake variable. If you want to run tests that execute hpp robot loading, be sure to have the package romeo_pinocchio installed (it is just a package done done by hand for the moment)

[C++][unittest] Added unittest geom.cpp . It tests that we obtain the same results between Pinocchio and hpp-model-urdf for joints and geometry objects positions ; for collisions and distances between two geometry objects

[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