Write and read interface for a spatial inertia matrix which reference frames can not be changed.
A spatial inertial matrix is a 6 by 6 matrix that gathers both the angular and linear inertia
including the cross components. The form of the matrix depends on the frame in which it is
expressed. When the frame's origin coincides with the center of mass position and that its axes
are aligned with the principal directions of the inertia ellipsoid, the spatial inertia matrix
takes the following form:
/ Jx,x 0 0 0 0 0 \
| 0 Jy,y 0 0 0 0 |
I = | 0 0 Jz,z 0 0 0 |
| 0 0 0 m 0 0 |
| 0 0 0 0 m 0 |
\ 0 0 0 0 0 m /
where m is the total mass, and Jx,x, Jy,y, and
Jz,z are the moments of inertia around the axes x, y, and z.
When the frame in which the inertia is expressed is arbitrary, the spatial inertia takes the
following general form:
/ Jx,x Jx,y Jx,z 0 -mz my \
| Jx,y Jy,y Jy,z mz 0 -mx |
I = | Jx,z Jy,z Jz,z -my mx 0 |
| 0 mz -my m 0 0 |
| -mz 0 mx 0 m 0 |
\ my -mx 0 0 0 m /