[Fix] interpolate signal with only one value

[danibene]

Description

Interpolating a single value currently raises an error, causing tests here to fail, and I know this has also previously been an issue #740

Proposed Changes

I changed the signal_interpolate() function so that it returns a constant signal if only one x value is provided rather than raising an error.

We could alternatively check the length of the input to signal_interpolate() in rsp_rvt() where it is currently causing the error. But unless we can think of a situation in which a constant signal is not the desired output given only one x value, I feel like directly modifying signal_interpolate() makes more sense than checking the length of the input wherever signal_interpolate() is used?

Checklist

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• I have read the CONTRIBUTING file.
• My PR is targetted at the dev branch (and not towards the master branch).

[codecov-commenter]

Codecov Report

Base: 53.77% // Head: 53.77% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (8656a66) compared to base (a45f7d4).
Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #787   +/-   ##
=======================================
  Coverage   53.77%   53.77%           
=======================================
  Files         293      293           
  Lines       13298    13299    +1     
=======================================
+ Hits         7151     7152    +1     
  Misses       6147     6147           

Impacted Files	Coverage Δ
neurokit2/signal/signal_interpolate.py	63.04% <66.66%> (-1.41%)	⬇️
neurokit2/rsp/rsp_simulate.py	93.75% <0.00%> (-0.90%)	⬇️
neurokit2/signal/signal_fixpeaks.py	73.76% <0.00%> (+0.76%)	⬆️

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[DominiqueMakowski]

Merge when you want!