BUG: Saving and loading None values

[scottclowe]

When a variable set to None is saved to an npz file, it gets turned into numpy.ndarray. We now turn np.array(None) back into None when loading it, and also don't save None values to the npz files any more.

This was found when trying to save the matfile for results that had been loaded from cache for the functional notebook #169, so I added a unit test to catch this error.

[codecov]

Codecov Report

Merging #199 (e0ba45c) into master (e4972a9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   93.06%   93.09%   +0.02%     
==========================================
  Files           8        8              
  Lines         909      912       +3     
  Branches      188      189       +1     
==========================================
+ Hits          846      849       +3     
  Misses         32       32              
  Partials       31       31              

Flag	Coverage Δ
nbsmoke	63.92% <50.00%> (-0.11%)	⬇️
unittests	92.98% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
fissa/core.py	96.64% <100.00%> (+0.03%)	⬆️

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