Ability to have executable outside of the directory with other model files

[okenk]

In general we are running the same SS3 executable on many different model configurations which are in different subdirectories. Thus, there is no need to keep the same executable in each configuration's subdirectory, and indeed it may be better NOT to copy the executable to ensure that each configuration is run using the same executable. r4ss functions generally allow you to specify the full path of the executable. It would be great to also have that functionality in nwfscDiag.