Fix filename produced by extrap_woa test case

The filename now indicates that it is for January (`_jan`) and
includes a datestamp.

compass/ocean/tests/utility/extrap_woa/extrap_step.py

@@ -1,4 +1,5 @@
 import os
+from datetime import datetime
 from functools import partial
 from multiprocessing import Pool
 
@@ -48,7 +49,9 @@ def setup(self):
         """
         Determine the output filename
         """
-        self.woa_filename = 'woa23_decav_0.25_extrap.nc'
+        now = datetime.now()
+        datestring = now.strftime("%Y%m%d")
+        self.woa_filename = f'woa23_decav_0.25_jan_extrap.{datestring}.nc'
         self.add_output_file(self.woa_filename)
 
     def run(self):