Pep8, pyflakes and MFDataset

@birdage I am using your notebook a lot for SECOORA (thanks!).  Here are some of the modifications I made, in case you are interested, to your version.  Most of the modification as purely aesthetic, like pep8 compliance and pyflakes fixes.  The only one that is worth noticing is the use of `netcdf4.MFDataset` to download the data.