Fixing definition of the beta.alpha_z operator

After discussions with Konstantin Gaul, a bug was found in the definition of the beta.alpha_z operator. In addition, the signs of beta.alpha_x and beta.alpha_y operators is changed in this MR.

In addition, some changes in the documentation are proposed.

Extra: Test added for the calculation of Wd (molecular enhancement factor of the electron electric dipole moment) using FSCC.