EAMxx: check input nc files format during buildnml

Debugging of issue #7341 took a considerable amount of time by @ndkeen @dqwu and others. The culprit (IIUC) was that one of the input files had the wrong nc format (netcdf4 instead of cdf5). While usually this should work (or provide a clear error from scorpio), in this case the error was weird.

I propose to add a check during eamxx buildnml run, to ensure that any nc file has cdf5 format. If the file does not exist, return without error, since buildnml runs many times, and the input download phase of CIME may have not run yet. If the file is present, and the format is NOT cdf5, error out with an informative message telling the user what to do. Besides avoiding the problem as in the linked issue, this would prevent wasting time in the queue for nothing...