Default dtype of pandas.Series is now a categorical

[raphaelvallat]

Closes #125

This PR:

• changes the default dtype of hyp.hypno as a pandas.CategoricalDtype
• set the default return dtype from np.int64 to np.int16 in Hypnogram.as_int()
• fixes a few typos and improves docstrings

Hopefully this should be pretty straightforward to review. Let me know if you have any questions! We're so close :)

[codecov-commenter]

Codecov Report

Base: 92.57% // Head: 92.59% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (7083921) compared to base (9be559d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   92.57%   92.59%   +0.01%     
==========================================
  Files          23       23              
  Lines        3112     3119       +7     
==========================================
+ Hits         2881     2888       +7     
  Misses        231      231              

Impacted Files	Coverage Δ
yasa/plotting.py	96.98% <ø> (ø)
yasa/hypno.py	98.58% <100.00%> (+0.01%)	⬆️
yasa/tests/test_hypno.py	100.00% <100.00%> (ø)
yasa/tests/test_hypnoclass.py	100.00% <100.00%> (ø)
yasa/tests/test_plotting.py	100.00% <100.00%> (ø)

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[remrama]

Sweeeeet. Approve 🚀

This was a great idea I'm so glad you considered this before release.

I also added some comments that address the last few that were still hanging on #124 when it was merged.