[Feature] add min, max, 80th & 20th percentiles as HRV features

[danibene]

Add the minimum, maximum, 20th percentile, and 80th percentile NN as features in the hrv_time function.

• I have read the CONTRIBUTING file.
• My PR is targeted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)

[welcome]

Thanks for opening this pull request! We'll make sure it's perfect before merging 🤗
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[DominiqueMakowski]

@danibene Great, thanks, very clean PR :)

It's mostly good, but could you perhaps add one or two references in which they use these indices? You can add the full ref in the Reference section and the short form (refauthor, 1998) after the index (in the future we could probably add that to all of the indices but that will be WIP)

[danibene]

Thank you :)

I added two references for each but please feel free to add/remove/edit.

[DominiqueMakowski]

Also, consider adding yourself to the contributor list if you want

[codecov-commenter]

Codecov Report

Merging #627 (8619557) into dev (a30f76e) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #627      +/-   ##
==========================================
+ Coverage   82.54%   82.62%   +0.07%     
==========================================
  Files         235      235              
  Lines       11276    11289      +13     
==========================================
+ Hits         9308     9327      +19     
+ Misses       1968     1962       -6     

Impacted Files	Coverage Δ
neurokit2/hrv/hrv_time.py	99.05% <100.00%> (+0.97%)	⬆️
neurokit2/eeg/eeg_rereference.py	83.33% <0.00%> (-3.77%)	⬇️
neurokit2/ecg/__init__.py	100.00% <0.00%> (ø)
neurokit2/microstates/microstates_plot.py	84.84% <0.00%> (ø)
neurokit2/signal/signal_distort.py	96.47% <0.00%> (+0.04%)	⬆️
neurokit2/ecg/ecg_simulate.py	98.43% <0.00%> (+0.17%)	⬆️
neurokit2/hrv/hrv_nonlinear.py	97.22% <0.00%> (+0.43%)	⬆️
neurokit2/rsp/rsp_simulate.py	99.10% <0.00%> (+4.46%)	⬆️

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[DominiqueMakowski]

Looks great, thanks again @danibene !! will merge as soon as the checks pass

[welcome]

Congrats on merging your first pull request! 🎉🍾 We're looking forward to your next one!