Fix_PBs_orientations

Here I fix the calculation of principal orientations of the PBs. I used the same code as in the Polyhedra shape class, to define "positive" orientations and replace Lapack with Eigen to calculate eigenvalues/eigenvectors, as we are gradually replacing Lapack in general. Also, I add a small docs update and some further checks upon the initialisation of a PB, aimed to fix bad geometrical inputs.

Hope you are all doing well, Vasileios