barb plots show barbs outside range of valid data

[Joern Ungermann]

Plots using barbs show barbs outside the valid range of available data. E.g. the standard geopotential height and horizontal wind speed plot.

This is likely a matplotlib/basemap issue, but we should work around it.

[Reimar Bauer]

Mask values outside valid range in wind field interpolation for barbs

Fix issue #443

→ <<cset 9aaa5fd7c795>>