Improved TestMultibodyPerformance timing information

I refined the measurements made by this timing test to provide more information about timings for individual stage calculations. These are relevant for computations where velocities must be iterated while positions are unchanged (e.g. semi-implicit Euler), or when new forces are applied but states are unchanged (e.g. Gazebo's application of discrete forces between steps). I also separated out timings for articulated body inertia calculations since those are the most expensive single computation in Simbody and could be substantially improved.