Warn instead of raise error if SpatialSeries has more than 3 columns

[rly]

Motivation

Fix #1479

Checklist

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• Have you checked our Contributing document?
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[codecov]

Codecov Report

Merging #1480 (de6b4f7) into dev (c9ea3f2) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1480      +/-   ##
==========================================
+ Coverage   78.18%   78.28%   +0.10%     
==========================================
  Files          37       37              
  Lines        2759     2772      +13     
  Branches      461      465       +4     
==========================================
+ Hits         2157     2170      +13     
  Misses        522      522              
  Partials       80       80              

Impacted Files	Coverage Δ
src/pynwb/behavior.py	100.00% <100.00%> (ø)

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[bendichter]

LGTM