[C++][algo] algorithm updateCollisionGeometry now optionally executes algorithm geometry to compute positions before updating collision objects's positions

[C++][Geom] distances results for computeDistances is not always cleared but replaced by a vector with empty results. The initialization of collision pairs is now  always done in constructor.

[benchmark] Added benchmark of atomic computations with geometry objects such as updating global positions, determine if two objects are in collision, or compute the distance between two objects

[Major][C++][Python]Breaking retrocompatibilty. Added the possibility to choose root joint when creating a model (in C++) or a robot (in Python). May be a check can be done on the value of root parameter in python if it's not None

[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

Conflicts:

	benchmark/timings.cpp

Scores:
RNEA = 		4.5206 us
CRBA = 		5.29557 us
Cholesky = 	2.33944us
Jacobian = 	3.6358 us
COM+Jcom = 	3.95835 us