Pysat

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* Expanded coverage in tutorial documentation
* Expanded test coverage for pysat.Meta()
* Improved robustness of Meta __setitem__
* Updated C/NOFS VEFI method to exempt empty file errors
* Updated C/NOFS VEFI download method to remove empty files
* Updated C/NOFS VEFI instrument module to use metadata from CDF file
* Updated superdarn cleaning method to remove empty velocity frames
* Updated Instrument download method to update bounds if bounds are default
* Updated C/NOFS IVM download method to remove empty files
* Updated C/NOFS IVM instrument module to use metadata from CDF file
* Performance improvements to seasonal occurrence probability
* Improved docstrings

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* Removed spacepy and netCDF from setup.py requirements. Both of
these packages require non-python code to function properly.
pysat now builds correctly as determined by travis-cl.
Installation instructions have been updated.

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* Added information to docstrings.
* Expanded unit test coverage and associated bugs.
* Changed signature for pysat.Instrument, orbit information
condensed into a single dictionary. pad changed from a boolean
to accepting a pandas.DateOffest or dictionary.
* Changed doy parameter in create_datetime_index to day.
* Changed Instrument.query_files to update_files
* Improved performance of cnofs_ivm code