DOC: Add default values to docstrings

[scottclowe]

Put defaults in docstrings.

c.f. https://numpydoc.readthedocs.io/en/latest/format.html#parameters

[codecov]

Codecov Report

Merging #232 (0d4b137) into master (b47e5aa) will increase coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
+ Coverage   92.45%   92.74%   +0.28%     
==========================================
  Files           8        8              
  Lines         941      937       -4     
  Branches      197      196       -1     
==========================================
- Hits          870      869       -1     
+ Misses         34       33       -1     
+ Partials       37       35       -2     

Flag	Coverage Δ
nbsmoke	65.95% <ø> (+0.06%)	⬆️
unittests	92.31% <ø> (+0.28%)	⬆️

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

Impacted Files	Coverage Δ
fissa/core.py	95.33% <ø> (+0.81%)	⬆️
fissa/deltaf.py	100.00% <ø> (ø)
fissa/neuropil.py	91.66% <ø> (ø)
fissa/roitools.py	90.22% <ø> (ø)

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