Possibly leaking figures when making salt frames.

We create these in `ThreddsFrameSource.salt_frame()`. It looks like we're cleaning up, but this warning still appears.

```
/home/john/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/matplotlib/pyplot.py:424: RuntimeWarning: More than 20 figures have been opened. Figures create
d through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see th
e rcParam `figure.max_open_warning`).
```