Deprecate alternative ways of computing inter-spike-intervals [vcs: #minor]

[anilbey]

Motivation

Having 3 different implementations for computing the inter-spike-intervals (a commonly used feature) is causing lots of inconsistencies. This PR reduces those three to only a single implementation.

Changes

• Reduce 3 alternative implementations to get ISIs into 1.
• "all_ISI_values" is recommended, "ISI_values" and "ISIs" are deprecated.
• The features depending on "LibV1:ISI_values" are moved to Python and now they depend on "all_ISI_values".
• BUGFIX: single_burst_ratio, irregularity_index, burst_mean_freq, interburst_voltage features were ignoring the first two ISIs instead of one when the ignore_first_ISI was set.
• Added new feature: inv_ISI_values that computes and returns all of the inverse isi values. Feature documentation? #208
• Drop python3.8 from the tox and gh-actions since scipy dependency's new version supports between py39-312.
  The users with python3.8 can still install it manually (the setup.py does not prevent it).

Checklist:

• [ X ] Unit tests are added to cover the changes.
• [ X ] The changes are mentioned in the documentation
• [ X ] CHANGELOG.md is updated (by default the change will increment the patch version, to increment major or minor changes add #major or #minor to the PR description)

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (1c02c12) 66.22% compared to head (53cbea3) 68.49%.

Files	Patch %	Lines
efel/pyfeatures/isi.py	97.43%	6 Missing ⚠️
efel/cppcore/Utils.cpp	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   66.22%   68.49%   +2.27%     
==========================================
  Files          33       36       +3     
  Lines        6504     6446      -58     
  Branches     2279     2087     -192     
==========================================
+ Hits         4307     4415     +108     
- Misses        273      275       +2     
+ Partials     1924     1756     -168     

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[ilkilic]

Well done! looks good to me