metgrid: print warning when no upper-level data found

[bbrashers]

I recently royally messed up a VTABLE, and accidentally removed all the lines for upper-level data. I was using ERA5 inputs, and had downloaded separate GRIB files for surface-level and upper-level data. I ran ungrib and metgrid, which gave no error messages, but the output met_em*.nc files were lacking the dimension num_metgrid_levels.

Perhaps metgrid should issue a warning when no upper-level output from ungrib is found - it might be worth the few lines of code it would likely take to save some confusion.

See https://forum.mmm.ucar.edu/phpBB3/viewtopic.php?f=34&t=8944&e=1&view=unread#unread