generic get_plot_data in base.py and update prepostfit code to get hdi

Author: lpoug

causalpy/experiments/prepostfit.py

@@ -310,6 +310,7 @@ def get_plot_data_bayesian(self, hdi_prob: float = 0.94) -> pd.DataFrame:
         if isinstance(self.model, PyMCModel):
             pre_data = self.datapre.copy()
             post_data = self.datapost.copy()
+            # PREDICTIONS
             pre_data["prediction"] = (
                 az.extract(self.pre_pred, group="posterior_predictive", var_names="mu")
                 .mean("sample")