Movie Geneator improvement

The main addition to this release is a more sophisticated plot_movie_generator().

• It now includes another yield statement after the data is loaded.
• This generator is wrapped with a start_generator decorator that progresses plot_movie_generator() to the first yield. This allows the generator function to raise errors regarding the data before the loop over the frames.
• At this point, the user can call plot_movie_generator.send('get_frame_data') to ask the generator to yield a NamedTuple of the times and frames arrays to be used in the movie. This enables the user to get the frames at once to analyze, for example, the mean auroral intensity along a satellite track, before the loop over individual frames.
• From the user point-of-view, the .send() command is optional, and the plot_movie_generator() is mostly backwards compatible (except the exception handling).

Updated the dependencies

This patch release specifies a more strict cdflib version. Also I added the example scripts to examples/ and into README.md

plot_movie functionality update

A few changes to the plot_movie.py module including:

• The frames are automatically saved to the /movies/frames/... directory
• Removed the clean_pngs kwarg.
• Added test_plot_movie.py that runs the examples in plot_movie.py There are no explicit tests, but a check to make sure the entire program runs.
• Changed the overwrite_ouput -> overwrite kwarg.

Import Bug Fix

Squashed a bug that didn't package the files in download and examples folders.