Fix Ensemble-Stat bug for handling missing observation data for the "first" field.

[JohnHalleyGotway]

This is a bug that was identified via MET-Help:
https://rt.rap.ucar.edu/rt/Ticket/Display.html?id=93937

When processing the "first" field in the "fcst.field" list, Ensemble-Stat initializes the output files. However, your observation file contains no data for "TMP/Z2". So there are zero pairs and it doesn't initialize the output files correctly. Your observations do contain data for "TMP/P850" so putting that first in the list causes the output files to be initialized correctly.

[JohnHalleyGotway]

The fix is a one-liner.
Replace: if(i == 0) setup_txt_files();
With: setup_txt_files();
Just call setup_txt_files() for each vx task. The first thing it does is check to see if the output files have already been initialized. And if so, it simply returns.