Dev - Symbolic dynamics HRV parameters added

[mrosol]

Description

This PR aims at adding calculation of symbolic dynamics HRV parameters [1-5]

[1] Cysarz, D., Edelhäuser, F., Javorka, M., Montano, N., and Porta, A. (2018). On the relevance of symbolizing heart rate variability by means of a percentile-based coarse graining approach. Physiol. Meas. 39:105010. doi: 10.1088/1361-6579/aae302
[2] Cysarz, D., Porta, A., Montano, N., Leeuwen, P. V., Kurths, J., and Wessel, N. (2013). Quantifying heart rate dynamics using different approaches of symbolic dynamics. Eur. Phys. J. Spec. Top. 222, 487–500. doi: 10.1140/epjst/e2013-01854-7
[3] Wessel, N., Malberg, H., Bauernschmitt, R., and Kurths, J. (2007). Nonlinear methods of cardiovascular physics and their clinical applicability. Int. J. Bifurc. Chaos 17, 3325–3371. doi: 10.1142/s0218127407019093
[4] Porta, A., Tobaldini, E., Guzzetti, S., Furlan, R., Montano, N., and Gnecchi-Ruscone, T. (2007). Assessment of cardiac autonomic modulation during graded head-up tilt by symbolic analysis of heart rate variability. Am. J. Physiol. Heart Circ. Physiol. 293, H702–H708. doi: 10.1152/ajpheart.00006.2007
[5] Gąsior, J. S., Rosoł, M., Młyńczak, M., Flatt, A. A., Hoffmann, B., Baranowski, R., Werner, B. (2022). Reliability of Symbolic Analysis of Heart Rate Variability and Its Changes During Sympathetic Stimulation in Elite Modern Pentathlon Athletes: A Pilot Study. Front. Physiol. 13, doi: 10.3389/fphys.2022.829887

Proposed Changes

I added neurokit2/hrv/hrv_symdyn.py and changed the following files neurokit2/hrv/hrv.py, neurokit2/hrv/__init__.py, tests/tests_hrv.py, docs/functions/hrv.rst

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• [+] 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.
• [NA] 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]

@Tam-Pham kind bump