Revert "MAINT: address pandas deprecation warning"

This reverts commit dca4e76.

src/pawian/data.py

@@ -142,7 +142,7 @@ def rho2(self) -> pd.DataFrame:
         """Compute a dataframe containing the square sum of the 3-momenta."""
         p3_squared = self.p_xyz**2  # type: ignore[operator]
         if self.has_particles:
-            return p3_squared.T.groupby(level=0).sum()
+            return p3_squared.groupby(axis=1, level=0).sum()
         return p3_squared.sum(axis=1)
 
     @property