[Docs] ecg_process: improve documentation and readability

[jchromik]

Description

This PR aims at updating and improving the documentation and code readability of the ecg_process() function.

Proposed Changes

This PR does not change any external behaviour of the function. Just refactoring and documentation.

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. (ISSUE/PLEASE NOTE: The section Structure and code says that "The maximum line length is 100 characters." but in the section Run code checks the line length is specified as 120 characters.)
• My PR is targetted 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)

[welcome]

Thanks for opening this pull request! We'll make sure it's perfect before merging 🤗
Make sure to read the contributing guide. Also, if you think that your contribution is worthy of it, you can consider adding yourself to the Contributors list (feel free to ask us if you have any doubts).

[DominiqueMakowski]

Hi @jchromik sorry I completely missed that, we'll review shortly :) thanks a lot!

[danibene]

Thank you for this @jchromik ! I left some comments

[danibene]

Thank you very much @jchromik ! @DominiqueMakowski feel free to merge

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (a802b57) 55.23% compared to head (bfbf012) 55.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #871      +/-   ##
==========================================
+ Coverage   55.23%   55.24%   +0.01%     
==========================================
  Files         297      297              
  Lines       13917    13916       -1     
==========================================
+ Hits         7687     7688       +1     
+ Misses       6230     6228       -2     

Files Changed	Coverage Δ
neurokit2/ecg/ecg_process.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[welcome]

Congrats on merging your first pull request! 🎉🍾 We're looking forward to your next one!