Generalize allowed input file names

I do not see a better way than to ask the user to provide the template
of basename as an additional argument. We cannot do as in
`Convert_Matlab/inst_eddies_v6.py`, that is try a list of templates,
because here the file names could be anything.

Inst_eddies/inst_eddies_in.py

@@ -14,6 +14,7 @@ parser = argparse.ArgumentParser()
 parser.add_argument("first_file",
                     help = "NetCDF file containing SSH and velocity at a "
                     "single date")
+parser.add_argument("date_format", help = "format of basename")
 parser.add_argument("-l", "--last_date", help = "%%Y-%%m-%%d")
 parser.add_argument("-b", "--bbox", nargs=4, type = float,
                     metavar = ("xmin", "xmax", "ymin", "ymax"))
@@ -29,9 +30,7 @@ if not os.access(inst_eddies_exe, os.X_OK):
 
 if os.access("SHPC_all_dates", os.F_OK): shutil.rmtree("SHPC_all_dates")
 dirname, basename = path.split(args.first_file)
-my_date = datetime.datetime.strptime(basename,
-                                     "dt_global_allsat_phy_l4_%Y%m%d"
-                                     "_20190101.nc").date()
+my_date = datetime.datetime.strptime(basename, args.date_format).date()
 nc_file = args.first_file
 nco_instance = nco.Nco()
 
@@ -89,7 +88,7 @@ while True:
 
     my_date += datetime.timedelta(1)
     if my_date > last_date: break
-    basename = my_date.strftime("dt_global_allsat_phy_l4_%Y%m%d_20190101.nc")
+    basename = my_date.strftime(args.date_format)
     nc_file = path.join(dirname, basename)
 
 os.remove("unpacked.nc")