Adds the universe mass to total mass and a unittest for a robot without joints

Pinocchio ignores the mass attached to the universe joint when calculating the total mass of a robot.

So far, we have been assuming the "mass" of the robot is the "total mass of the system" -- and the difference went unnoticed. This MR adds the mass prior to the first joint to the robot and includes a unittest to check not only that, but also the general base functionalities of the PinRobot if there are no joint at all.