Add --backfill option to download_weather worker

[douglatornell]

The download_weather worker presently raises and error and exits if the destination directory it is to write to already exists when lib.mkdir() is called for it. While that is good to avoid accidental overwrites, there are situations during recovery from automation issues that it is desirable to allow the overwite. Specifically, to recover from a missed HRDPS forecast download we want to start the crop_gribs worker, which creates the destination directory, then start download_weather so that cropping happens as the download progresses rather than having to wait until after the download finishes to start cropping.

Adding a boolean --backfill option that defaults to False and passing its value to lib.mkdir() via the exist_ok argument will facilitate that use case.