Efel

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- Refactored LibV1, LibV2, LibV3, LibV5 into more meaningful categories: SpikeEvent, SpikeShape and Subthreshold.

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- Fixed impedance feature (frequency index was inconsistent with smooth Z)

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- Adding AP_height to documentation

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- Reduce 3 alternative implementations to get ISIs into 1.
- "all_ISI_values" is recommended, "ISI_values" and "ISIs" are deprecated.
- The features depending on "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 when the ignore_first_ISI was set.
- Added new feature: inv_ISI_values that computes and returns all of the inverse isi values.

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- Type annotate api.py's functions.
- Deprecate camel case function names in api.py.
- Start using same requirements_docs.txt in readthedocs and tox.
- Enable autodoc and typehints in the API documentation.
- Fix docstring errors in the io module.
- Add changelog to the documentation.

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- New feature: phaseslope_max