Add computation examples on matrix multiplication VS for loop

[pomerlef]

In lessons/transformations_2d/3-lesson_rigid.ipynb add a script that compute the time of moving a point cloud using matrix mult. vs for loop.

Aslo, in Section "2. Combining basic transformations", review that question:

\begin{aligned}
\mat{P}' = \mat{R}_2 (\mat{R}_1 \mat{P} + \mat{t}_1 ) + \mat{t}_2
\textcomma
\end{aligned}

as it only work for a point, not for matrices.

[DomBaril]

Added the computational difference script, not sure what is needed for the equation as I think a pseudo-code with a for loop for every point would work.

[pomerlef]

For the equation, there was a mistake. It needs to be

\begin{aligned}
\mat{p}' = \mat{R}_2 (\mat{R}_1 \mat{p} + \mat{t}_1 ) + \mat{t}_2
\textcomma
\end{aligned}

With a lowercase p. Check the text to make sure it fit the change of equation.

We cannot sum a matrix with a vector: \mat{P} + \mat{t}.

[DomBaril]

Fixed in next commit.