DOC: Improve core docstrings

[scottclowe]

• Move parameters from __init__ to Experiment docstring.
• Add Attributes section covering output attributes.
• Change the Sphinx settings to use numpydoc instead of napolean, which generates better attributes sections in particular.
• Change df/f0 to Δf/f₀.
• Add link to FISSA paper in Experiment docstring
• Add versionadded directive to the new methods introduced in RF+ENH+BUG: Use npz for cache; store means #177.

[codecov]

Codecov Report

Merging #204 (be90cd6) into master (b03adf1) will decrease coverage by 1.27%.
The diff coverage is 7.14%.

❗ Current head be90cd6 differs from pull request most recent head 5c447ee. Consider uploading reports for the commit 5c447ee to get more accurate results

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   93.31%   92.03%   -1.28%     
==========================================
  Files           8        8              
  Lines         927      942      +15     
  Branches      194      199       +5     
==========================================
+ Hits          865      867       +2     
- Misses         32       45      +13     
  Partials       30       30              

Flag	Coverage Δ
nbsmoke	65.18% <7.14%> (-0.84%)	⬇️
unittests	91.71% <7.14%> (-1.27%)	⬇️

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Impacted Files	Coverage Δ
fissa/neuropil.py	91.42% <ø> (ø)
fissa/roitools.py	88.98% <ø> (ø)
fissa/core.py	93.29% <7.14%> (-3.84%)	⬇️

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