Create `perf_report.csv` in the slice directory

So that several processes running `inst_eddies.py` for different
slices can run in parallel.

Inst_eddies/inst_eddies.py.in

@@ -42,7 +42,8 @@ def loop_inst_eddies(files, bbox, d, my_slice):
         if os.access(my_dir, os.F_OK): shutil.rmtree(my_dir)
         os.makedirs(my_dir)
 
-    perf_report = open("perf_report.csv", "w", newline = '')
+    perf_report = open(f"SHPC/Slice_{my_slice}/perf_report.csv", "w",
+                       newline = '')
     writer = csv.writer(perf_report, lineterminator = "\n")
     writer.writerow(["", "s", "s"]) # units
     writer.writerow(["date", "elapsed NCO", "elapsed Fortran"]) # long names