FileSet description omit time if daily

If a dataset has a period longer than a day, omit the timestamp from the
automatically generated description. Makes logs a bit briefer.

csppfetch/__init__.py

@@ -658,8 +658,12 @@ class Downloader:
 
                 expected = self.is_expected(time)
 
+                fmt_time = time.strftime("%Y-%m-%d")
+                if self.period < timedelta(days=1):
+                    fmt_time += time.strftime("%H:%M:%S")
+
                 priority_desc = f'"{priority}"' if priority != "" else '"" (empty string)'
-                description = f"{self.name} for {time} at priority {priority_desc}"
+                description = f"{self.name} for {fmt_time} at priority {priority_desc}"
 
                 fs = FileSet(urls_to_files, expected, description)
                 filesets.append(fs)