[Fix] Calculation of number of NaNs in signal_fixpeaks()

[danibene]

Description

This PR aims to improve the previous PR: https://github.com/neuropsychology/NeuroKit/pull/651 See also the related issue: https://github.com/neuropsychology/NeuroKit/issues/650

Proposed Changes

I changed the neurokit method of the signal_fixpeaks() function so that the number of NaNs inserted is computed as follows:

1. Rounding rather than taking the floor when getting an integer value for the number of NaNs (calculated by dividing large gaps by the mean interval)
2. Calculating the mean interval based on the original interval values and not the standardized interval values in the case that relative_interval_max is given.

Checklist

• [x] I have read the CONTRIBUTING file.
• [x] My PR is targeted 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 got the following error, but I'm just going to ignore it if that's okay, since I don't think I changed anything that should cause this? astroid.exceptions.TooManyLevelsError: Relative import with too many levels (2) for module 'signal_fixpeaks' Could also be the fact that I have a file called signal_fixpeaks.py in a different folder.
• [ ] I have added the newly added features to News.rst (if applicable)

[DominiqueMakowski]

Minor style issue https://github.com/neuropsychology/NeuroKit/runs/7420854905?check_suite_focus=true#step:8:23 to fix before we can merge!

[danibene]

Minor style issue https://github.com/neuropsychology/NeuroKit/runs/7420854905?check_suite_focus=true#step:8:23 to fix before we can merge!

@DominiqueMakowski if I understood correctly it's an issue with signal_detrend()? Should I modify that file?

[DominiqueMakowski]

oops yeah my bad I introduced that just now 🙈 I'll fix it on dev then

[codecov-commenter]

Codecov Report

Merging #679 (8cdac4b) into dev (597594b) will increase coverage by 0.04%. The diff coverage is 78.26%.

@@            Coverage Diff             @@
##              dev     #679      +/-   ##
==========================================
+ Coverage   52.47%   52.52%   +0.04%     
==========================================
  Files         277      277              
  Lines       12508    12502       -6     
==========================================
+ Hits         6564     6567       +3     
+ Misses       5944     5935       -9     

Impacted Files	Coverage Δ
neurokit2/signal/signal_fixpeaks.py	73.00% <78.26%> (+0.14%)	:arrow_up:
neurokit2/eda/eda_eventrelated.py	83.67% <0.00%> (+2.04%)	:arrow_up:
neurokit2/rsp/rsp_simulate.py	99.10% <0.00%> (+5.35%)	:arrow_up:

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[DominiqueMakowski]

@danibene apologies for the delay - it's been a busy month! I'm gonna merge this once it passes the checks :)

[danibene]

@DominiqueMakowski nice, thanks :)