This PR introduces the ability to combine inertial properties of different objects, returning the equivalent inertial properties as if the objects were welded together.
The calculation is as follows:
Add the masses: new_mass = mass1 + mass2
Compute the new center of mass location: pose = mass1*pose1 + mass2*pose2
Add the moment of inertias in the base frame: MOI = MOI1 + MOI2
Parallel axis theorem: account for offset of each mass from new center of mass (see code)
I also fixed a code_check complaint in Plane.hh and added a default value to the Quaternion argument in the SetFromBox functions, which allows that function to be called more concisely when no rotation is desired (api / abi is fine).