CHG: chg/add quantiles methods for wfpt, and fix _plot_posterior_quantil...

remove Wfpt.quantiles
added Wfpt.empirical_quantiles - compute the quantiles over the stochastic's value
added Wfpt.theo_quantiles - compute the theoretical quantiles based on  stochastic's parents

fix: _plot_posterior_quantiles used the theoretical quantiles, but it should have been generating Wfpt values, based on the trace of the parents and then compute the quantiles of the new generated values.