Translate the cross-sections functions in Julia

[egavazzi]

With this PR, the whole setup is finally fully translated in Julia.

Effects:

• Faster setup. The first time the code is called, we go from 13.5s with the Matlab code to 11s with the new one, which is not a big improvement. But for subsequent calls (so after everything has been compiled during the first call), we go from 8s to 0.01s. This is a nice when we run a batch of simulations (multiple simulations one after the other).
• Less bugs and easier startup. Setting up everything so that Julia can call Matlab code can be tedious. For example some versions of Matlab don't work with the MATLAB.jl package on some OS. Users installing the code don't need to bother with that anymore. The Matlab session called by Julia has also been sometimes throwing segmentation faults (randomly it seems), which has been particularly annoying when running some batch of simulations. This should hopefully not happen anymore.