new functionality, download from inputdata without a case

[jedwards4b]

The CUPiD postprocessing team would like to be able to download files from input data without having created a case. NCAR/CUPiD#269

[mnlevy1981]

Thanks, @jedwards4b! I just want to note that this is not a very high priority for us -- there are only one or two files in inputdata that we want to share at the moment and we can certainly help users copy them by hand for the time being. I think there is still an open question about whether observational datasets used for comparisons with model output belong in inputdata or somewhere else; the functionality being proposed here will definitely be useful for CUPiD if we move some of those obs to inputdata.