Save `time.perf_counter()` in t1

So we do not call `time.perf_counter()` a second time after a simple
assignment.

Inst_eddies/inst_eddies.py.in

@@ -35,8 +35,9 @@ def process_1_file(bbox, nco_instance, nc_file, cropped_fname, unpacked_fname,
         nc_file = cropped_fname
 
     nco_instance.ncpdq(nc_file, output = unpacked_fname, options = ["--unpack"])
-    elapsed_NCO = time.perf_counter() - t0
-    t0 = time.perf_counter()
+    t1 = time.perf_counter()
+    elapsed_NCO = t1 - t0
+    t0 = t1
     subprocess.run([inst_eddies_exe, unpacked_fname, unpacked_fname],
                    check = True, input = f"&dates_nml date = {d}, "
                    f"slice = {my_slice}/\n", text = True)