[Fix] use signal_filter() in ecg_clean() so that certain methods do not fail with low sampling rate

[danibene]

Description

Since the ecgzeemod2012 method was failing with a low sampling rate, this PR aims at refactoring several ECG cleaning methods so that they use signal_filter rather than scipy directly. See https://github.com/neuropsychology/NeuroKit/issues/685

Proposed Changes

I changed the signal_filter() function so that it includes a method butterworth_zi calling scipy.signal.sosfilt rather than scipy.signal.sosfiltfilt, and called this method in ecg_clean() so that the output does not change from the existing implementation. Let me know if you think we should do otherwise (I'm not actually sure what the original papers used as filters but I thought that's a separate issue...)

Checklist

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

• [x] I have read the CONTRIBUTING file.
• [x] My PR is targetted at the dev branch (and not towards the master branch).
• [x] 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: 52.65% // Head: 52.64% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (632eb1e) compared to base (262cc00). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #709      +/-   ##
==========================================
- Coverage   52.65%   52.64%   -0.02%     
==========================================
  Files         277      277              
  Lines       12588    12583       -5     
==========================================
- Hits         6628     6624       -4     
+ Misses       5960     5959       -1     

Impacted Files	Coverage Δ
neurokit2/ecg/ecg_clean.py	91.93% <100.00%> (-1.40%)	:arrow_down:
neurokit2/signal/signal_filter.py	68.62% <100.00%> (+2.67%)	:arrow_up:
neurokit2/rsp/rsp_simulate.py	98.21% <0.00%> (-0.90%)	:arrow_down:
neurokit2/signal/signal_fixpeaks.py	73.76% <0.00%> (+0.76%)	:arrow_up:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.