fix(polargen): fix extension overwrite

src/airfoil_data/polargenerator.py

@@ -93,7 +93,7 @@ def cleanup(airfoil):
     for file in os.listdir(cwd):
         if file.startswith("Coordinates_"):
             safe_rename(file, airfoil + ".dat")
-        elif file.startswith("Polar_"):
+        elif file.startswith("Polar_") and not (file.endswith(".txt")):
             safe_rename(file, file + ".txt")