Support netCDF- and Zarr-based baseline and truth data #20

Currently, wxvx required GRIB-formatted baseline and truth (when gridded) data. It could also support netCDF and Zarr.

Unlike with GRIB, where individual messages can be pulled from remote or local GRIB files, netCDF will require copying full remote netCDF files locally, then extracting grids to write as individual, CF-compliant netCDF files to pass on to MET. The MET configs will need to be modified to read .nc files as well as .grib2 files.

IIRC individual Zarr grids can be remotely accessed.

Maybe behavior could be determined by the extension on baseline.url or truth.url, but GRIB files have a variety of extensions, so it's possible that we'll need a new config switch to specify the format. (On the other hand, netCDF files generally have a .nc extension, so maybe the behavior could be: Treat baseline.url as netCDF if it ends with .nc, and as GRIB otherwise. Similar for the standard .zarr extension.)