Fix potential namespace collision. [1303]

Owing to the fact that McIDAS-V auto-imports things like buildWindow
into the "__main__" namespace and because background.py had imported
"from java.awt import Dimension", having a user doing something like
the following will overwrite the Dimension class that buildWindow is
using:

from ucar.nc2 import Dimension
buildWindow()

Note that everything worked as expected if you did something like:
import background
from ucar.nc2 import Dimension
background.buildWindow()