[Feature] ECG inversion

[danibene]

Description

This PR aims at adding the ability to identify and correct ECG signal inversion. See also #558

Proposed Changes

I added the ecg_invert() function so that a user could optionally correct for signal inversion in the raw ECG data before using the other processing functions (cleaning, peak detection).

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 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)

[codecov-commenter]

Codecov Report

Base: 52.72% // Head: 52.76% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (58fe039) compared to base (a811797).
Patch coverage: 72.41% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #716      +/-   ##
==========================================
+ Coverage   52.72%   52.76%   +0.03%     
==========================================
  Files         279      280       +1     
  Lines       12643    12672      +29     
==========================================
+ Hits         6666     6686      +20     
- Misses       5977     5986       +9     

Impacted Files	Coverage Δ
neurokit2/ecg/ecg_peaks.py	100.00% <ø> (ø)
neurokit2/ecg/ecg_quality.py	26.31% <ø> (ø)
neurokit2/ecg/ecg_invert.py	71.42% <71.42%> (ø)
neurokit2/ecg/__init__.py	100.00% <100.00%> (ø)
neurokit2/eda/eda_eventrelated.py	81.63% <0.00%> (-2.05%)	⬇️

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[DominiqueMakowski]

I streamlined the example a bit and added a show option. Feel free to adjust/modify!

(Also, note that I added some

@suppress
plt.close()

After the plots: the reason for this is that basically, when sphinxruns the documentation and runs ipython on the example chuncks, it will create matplotlib figures. But it doesn't close them automatically, meaning that after 20 opened images it will error and say that there are too many images concurrently opened (and the documentation build will fail). So we need to close the plots that we create in the docstrings manually; and the @suppress tag is there to hide this line from the actual docs)

[danibene]

@DominiqueMakowski this all looks good to me, I'll go ahead and merge!