be_polyelectrolyte model needs proper validation check

[butler]

An error in the be_polyelectrolyte model was identified prior to release 4.2.0 and corrected. The error has been carried since 1998 and there was no quick and easy external validation of that change, partly because the implementation doesn't exactly follow the paper. It would be good if somebody could go through the paper and ensure that the code follows so that the disclaimer note in the docs can be removed.