Pre-compute col_tiers in DataLoader.build_entry_list() for speedup (

#523)

pre-compute col_tiers for speedup

src/pygama/flow/data_loader.py

@@ -505,6 +505,8 @@ def build_entry_list(
 
         # Find out which columns are needed for any cuts
         cut_cols = {}
+        # ... and pre-load which tiers need to be loaded to make the cuts
+        col_tiers_dict = {}
 
         for level in [child, parent]:
             cut_cols[level] = []
@@ -527,6 +529,9 @@ def build_entry_list(
                             and save_output_columns
                         ):
                             for_output.append(term)
+            col_tiers_dict[level] = self.get_tiers_for_col(
+                cut_cols[level], merge_files=False
+            )
 
         if save_output_columns:
             entry_cols += for_output
@@ -611,7 +616,7 @@ def build_entry_list(
                     if level in self.cuts.keys():
                         cut = self.cuts[level]
 
-                col_tiers = self.get_tiers_for_col(cut_cols[level], merge_files=False)
+                col_tiers = col_tiers_dict[level]
 
                 # Tables in first tier of event should be the same for all tiers in one level
                 tables = self.filedb.df.loc[file, f"{self.tiers[level][0]}_tables"]