[Feature] Add membership function options for fuzzy entropy

[DongCheninmuenster]

Description

I noticed a TODO about membership functions in utils_entropy.py when I tried to use this package for fuzzy entropy analysis. Therefore I added following 8 kinds of membership functions:

    "exp"    : exponential,
    "gauss"  : gaussian,
    "cgauss" : constgaussian,
    "bell"   : bell,
    "z"      : z,
    "trapez" : trapezoidal,
    "tri"    : triangular,
    "sig"    : sigmoid,

See:

Azami, H., Li, P., Arnold, S. E., Escudero, J., & Humeau-Heurtier, A. (2019b). Fuzzy Entropy Metrics for the analysis of Biomedical Signals: Assessment and comparison. IEEE Access, 7, 104833–104847. https://doi.org/10.1109/access.2019.2930625

Zheng, J., Jiang, Z., & Pan, H. (2018b). Sigmoid-based refined composite multiscale fuzzy entropy and t-SNE based fault diagnosis approach for rolling bearing. Measurement, 129, 332–342. https://doi.org/10.1016/j.measurement.2018.07.045

Changes

• Introduced a new func_name parameter for selecting the membership function.

• Modified the tolerance parameter to accept both scalar and two-element vector formats as the parameters of membership functions. Detailed definition can be found in the referenced papers above.

  • If it's a scalar, it refers to a threshold.
  • If it's a two-element vector, it refers to threshold and power respectively.

• To optimize memory usage, fuzzy entropy is calculated using block matrices. (block size is 10 in default)

Testing

Compared the result with Azami's MATLAB version: FuzzyEntropy_Matlab

Example

print(nk.entropy_fuzzy(series, func_name="exp", tolerance=[0.0018,4]))
print(nk.entropy_fuzzy(series, func_name="gauss", tolerance=0.1253))
print(nk.entropy_fuzzy(series, func_name="cgauss", tolerance=0.0903))
print(nk.entropy_fuzzy(series, func_name="bell", tolerance=[0.1414,2]))
print(nk.entropy_fuzzy(series, func_name="z", tolerance=0.1309))
print(nk.entropy_fuzzy(series, func_name="trapez", tolerance=0.1286))
print(nk.entropy_fuzzy(series, func_name="tri", tolerance=0.3))

Checklist

• 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)

[DongCheninmuenster]

Some changes are just format changes modified by checks code