Fix Implementation of Sandwich Variance estimation in PLN

[jchiquet]

Bastien's implementation in https://bbatardiere.pages.mia.inra.fr/pyplnmodels/, described in
https://arxiv.org/abs/2411.08524 works quite well and we should reach the same kind of result in our R/C++ implementation.

Assessing uncertainties of variational estimator of the regression coefficients, at least, is a great asset for PLNmodels compared to other similar models / implementation.