Add check before plotting wind barbs

Fixes Unidata#2785.

Currently, BarbPlot is happy to attempt to plot very large wind barbs,
but if the size becomes too large, memory usage becomes an issue,
leading to system hangs.  This commit adds a check to make sure the
wind barbs will not consist of so many pennants that memory could be
an issue.

src/metpy/plots/declarative.py

@@ -1660,9 +1660,15 @@ def _build(self):
 
         wind_slice = (slice(None, None, self.skip[0]), slice(None, None, self.skip[1]))
 
+        u_vals = u.values[wind_slice]
+        v_vals = v.values[wind_slice]
+        speeds = np.hypot(u_vals, v_vals)
+        if np.median(speeds) > 50000:
+            raise ValueError('Too many large wind barbs to plot')
+
         self.handle = self.parent.ax.barbs(
             x_like[wind_slice], y_like[wind_slice],
-            u.values[wind_slice], v.values[wind_slice],
+            u_vals, v_vals,
             color=self.color, pivot=self.pivot, length=self.barblength, zorder=2, **kwargs)