Always store metadata rotation as a string

src/prep.py

@@ -80,7 +80,7 @@ def main(argv = None):
     f.check_dest_metadata({ "prep_raw_source": f.src.path,
                             "prep_sinefit_source": sinefit.path,
                             "prep_column": args.column,
-                            "prep_rotation": rotation })
+                            "prep_rotation": repr(rotation) })
 
     # Run the processing function on all data
     f.process_numpy(process, args = (client_sinefit, sinefit.path, args.column,