Fix verbose gnuplot

[GillySpace27]

When the code is run, the wind speed calculation prints hundreds of lines of gnuplot errors like the following:

(in cleanup) (in cleanup) at /Users//perl5/lib/perl5/PDL/Graphics/Gnuplot.pm line 7630, <AUTOLOAD_FILE> line 2760 during global destruction.'''

However, when run a second time, the errors don't appear, so something in the preceding code is seems to be setting up gnuplot to fail.

[GillySpace27]

I cannot for the life of me get this fixed. I've tried suppressing different kinds of warnings, and even running this routine with a different scope, but the printouts won't go away.