Revert "Removing Levels Index Variable Within Atomic Data Preperation (…

…#1957)"

This reverts commit 507c50f.

tardis/io/atom_data/base.py

@@ -330,6 +330,10 @@ def prepare_atom_data(
 
         self.nlte_species = nlte_species
 
+        self.levels_index = pd.Series(
+            np.arange(len(self.levels), dtype=int), index=self.levels.index
+        )
+
         # cutting levels_lines
         self.lines = self.lines[
             self.lines.index.isin(
@@ -348,10 +352,22 @@ def prepare_atom_data(
             "level_number_upper"
         )
 
+        self.lines_lower2level_idx = (
+            self.levels_index.loc[tmp_lines_lower2level_idx]
+            .astype(np.int64)
+            .values
+        )
+
         tmp_lines_upper2level_idx = self.lines.index.droplevel(
             "level_number_lower"
         )
 
+        self.lines_upper2level_idx = (
+            self.levels_index.loc[tmp_lines_upper2level_idx]
+            .astype(np.int64)
+            .values
+        )
+
         if (
             self.macro_atom_data_all is not None
             and not line_interaction_type == "scatter"