Update RadMon satype and base files

[EdwardSafford-NOAA]

Update these fix files to reflect currently available sources including n21 instruments and removing n18 instruments:

• gdas_radmon_satype.txt
• gdas_radmon_base.tar

Ensure the RadMon website squares with these changes, leaving g18 sources available on the website until we lose them entirely.

[EdwardSafford-NOAA]

I've made the changes to the two fix files and have also incorporated a related change to the MkBase.sh script to avoid a tar error if a new source is added to the base.tar file rather than replacing an existing base file. Lastly I added a fix to the transfer.sh script that I've been working on to avoid wiping out the thumbnail png files on the server.

I'm going to let this run over the weekend to make sure it's all performing as intended and if so I'll issue a PR on Monday.

[EdwardSafford-NOAA]

This was recently done in #225.