MNT: Use ndarray for all outputs, not list for some

[scottclowe]

Because we make the first of each output type with list comprehension and then copy it with np.copy(), that means one output is a list of lists of x and the others are 2d numpy arrays of x.

This PR changes the odd-one-out so it is a numpy array as well.

[codecov]

Codecov Report

Merging #164 (4898354) into master (ab32a9e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   92.68%   92.71%   +0.02%     
==========================================
  Files           9        9              
  Lines         779      782       +3     
  Branches      158      158              
==========================================
+ Hits          722      725       +3     
  Misses         29       29              
  Partials       28       28              

Flag	Coverage Δ
unittests	92.58% <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	95.96% <100.00%> (+0.04%)	⬆️

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