TST: Extend core tests

[scottclowe]

• Generate downsampled test data for more of the example ROIs and TIFFs.
• Test on this experiment instead of the previous experiment with two ROIs and three TIFFs (instead of the previous version with only one ROI and one TIFF)
• Check the values of raw, roi_polys, means, mixmat, sep, result, deltaf_result, deltaf_raw match the expected values.
• We have a copy of the expected values for Python 2 and another for Python 3 because of changes to pickle which you can't turn back to the python 2 settings while using np.load to load npz files.

Closes #10.

[codecov]

Codecov Report

Merging #202 (80ce979) into master (0b6026d) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   93.20%   93.31%   +0.10%     
==========================================
  Files           8        8              
  Lines         927      927              
  Branches      194      194              
==========================================
+ Hits          864      865       +1     
  Misses         32       32              
+ Partials       31       30       -1     

Flag	Coverage Δ
nbsmoke	66.01% <ø> (ø)
unittests	92.98% <ø> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
fissa/core.py	97.12% <0.00%> (+0.31%)	⬆️

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