* One Definition Rule was still not enforced.
  * Declare function in header sample-model.hpp, implement them in
    sample-models.cpp
  * Add sample-models symbols to libpinocchio.

[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