Center-of-mass elimination

[ohno]

Add a function to eliminate center-of-mass motion after #2. The arguments may be the kinetic terms and the coordinate transformation matrix $\pmb{A}$:

$$\begin{pmatrix} \pmb{r}'_1 \\\ \pmb{r}'_2 \\\ \pmb{r}'_3 \\\ \end{pmatrix} = \begin{pmatrix} A_{11} A_{12} A_{13}\\\ A_{21} A_{22} A_{23}\\\ A_{31} A_{32} A_{33}\\\ \end{pmatrix} \begin{pmatrix} \pmb{r}_1 \\\ \pmb{r}_2 \\\ \pmb{r}_3 \\\ \end{pmatrix}.$$

[MartinMikkelsen]

Maybe use this