global_det: Threshold values in ush/global_det/plot_util.py

[malloryprow]

From Bugzilla 1586

The code evs_global_det_wave_grid2obs_plots_31days crashed with empty array error when ush/global_det/plot_util.py over filtered the wind data. The original threshold was set to 200, with new threshold of 500 introduced on 1.0.10 the code manages to keep the data. There is no guarantee that 500 always going to work.
The developer group is deciding on how to scientifically/logically decide on the threshold on next upgrade. This is a Bugzilla ticket to document and track progress on the problem.

For the record here is the line of code
stat_values = stat_values[stat_values<=500]

The lines in ush/global_det/plot_util.py are 1227 and 1529 that needed thought of how to handle.

Related to #534