Pyhrv

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- added new time domain HRV parameters -> geometrical parameters

* Triangular Index: _hrv.time_domain.triangular_index()_
* TINN: _hrv.time_domain.tinn()_
* Histogram (helper function powered by NumPy and Matplotlib): _hrv.time_domain._get_histogram()
* Geometrical Parameters (calls _tinn()_ and _triangular_index()_ as those parameters are rarely used alone): _hrv.time_domain.geometrical_parameters()_

- updated _hrv.time_domain.time_domain()_ to also return the new geometrical parameters
- added new nonlinear parameters -> sample entropy and detrended fluctuation analysis (DFA)

* Sample Entropy: _hrv.nonlinear_parameters.sample_entrop()_
* DFA: _hrv.nonlinear_parameters.dfa()_
* Both functions powered by the [nolds](https://github.com/CSchoel/nolds)

- updated _hrv.nonlinear_parameters.nonlinear()_ to also return the new nonlinear parameters
- added new tools function -> HRV Report
* Entirely new HRV report generator: _hrv.tools.hrv_report()_
* generates reports in .txt or .csv format

- added_ _check_fname()_ function to detect existing files for the generated HRV exports and HRV reports and automatically generate a new file name to avoid overwriting existing files (e.g. if 'Sample.txt' exists, it will 'increment' the new file name to 'Sample_1.txt' to avoid overwriting 'Sample.txt')

- added 'SampleReport.txt' and 'SampleReport.csv' to the './hrv/files/'
- updated 'SampleExport.json' in './hrv/files/'
- fixed minor fixed in some tools functions
- improved compatibility with Python 3
- updated example sections in each module

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- restructured entire repository
- tested functionality and validated results for the _time_domain_, _nonlinear_parameters_, and _tools_ modules
- fixed example scripts at the end of each module
- added new functions to the _tools_ module: _hrv_import()_,
_hrv_export_(), _check_input()_
- fixed bugs in the _tools.segmentation()_ function

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- time domain functions
- poincaré nonlinear functions
- basic tools for HRV functions