[Fix] prevent deprecation warning in signal_interpolate()

[danibene]

Description

This PR aims to prevent the following warning when running signal_interpolate():
DeprecationWarning: elementwise comparison failed; this will raise an error in the future.

Proposed Changes

I changed the signal_interpolate() function so that a line (that I think I added, by the way, sorry) using np.all now uses np.array_equal instead:
https://stackoverflow.com/questions/44574679/python-deprecationwarning-elementwise-comparison-failed-this-will-raise-an

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).

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (3d5ecfc) 53.82% compared to head (fae8a3e) 53.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #816      +/-   ##
==========================================
- Coverage   53.82%   53.81%   -0.01%     
==========================================
  Files         295      295              
  Lines       13799    13799              
==========================================
- Hits         7427     7426       -1     
- Misses       6372     6373       +1     

Impacted Files	Coverage Δ
neurokit2/signal/signal_interpolate.py	63.04% <100.00%> (ø)

... and 2 files with indirect coverage changes

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