Improve imcal ml benchmarks

benchmarks/imaging_shower_ML/scripts/epcut_scan.py

 """
     A script to scan the optimized cut on layer and E/p.
-    It scan all the possible ScFi layers (20 in the EPIC brycecanyon configuration)
-    The results give the best cut (highest pion rejection) on [layer, E/p] with a targeted electron efficiency
+    It groups [event, layer] and scan the E/p cut layer-wise (e.g., EPIC brycecanyon configuration has 16 ScFi layers)
+    The best 2D cut (highest rejection power) on [layer, E/p] that satisfies required effiiency will be given
 
     Chao Peng (ANL)
     2022/11/13