MNT: Suppress warnings

[scottclowe]

• Avoid implicit ragged numpy.ndarray creation when comparing shapes.
• Suppress unavoidable warnings about ragged numpy.ndarray when saving matfile.
• Suppress expected division by zero warnings during unit test suite.

[codecov]

Codecov Report

Merging #243 (de17ce6) into master (de1dfc0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
+ Coverage   93.72%   93.74%   +0.01%     
==========================================
  Files           8        8              
  Lines        1068     1071       +3     
  Branches      248      248              
==========================================
+ Hits         1001     1004       +3     
  Misses         34       34              
  Partials       33       33              

Flag	Coverage Δ
nbsmoke	59.75% <33.33%> (+0.11%)	⬆️
unittests	93.46% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
fissa/core.py	96.42% <100.00%> (+0.02%)	⬆️

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