Exit 0 if all input iceberg restarts are empty

[ceblanton]

Description
Checks to see if all input files are empty. If so, print a message to the user stating all is fine, and exit 0.

Fixes #358

How Has This Been Tested?
I tested it on empty input files and non-empty files.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[underwoo]

This will always be true as $combine_files hasn't yet been defined.

Please describe why you want this option here? If the number of files passed to this is zero, then it should be a failure.

[mlee03]

perhaps line 318 can be changed to if [ -s $out_file ]? that checks if the file exists and its size is > 0