color by columns that are 1d vectors

[Saksham20]

while plotting traces of image_masks from the PlaneSegmentation table, legend groups should only be made for those table columns that are 1d vectors. Columns for image_mask/pixel_mask/Centroid etc are 2d or 3d vectors, for which color by would not make sense. Additionally, for such columns there would be as many groups as no of rows.

[codecov]

Codecov Report

Merging #197 (bc6c909) into master (b2fee99) will decrease coverage by 1.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
- Coverage   67.72%   66.64%   -1.09%     
==========================================
  Files          32       32              
  Lines        2947     2947              
==========================================
- Hits         1996     1964      -32     
- Misses        951      983      +32     

Flag	Coverage Δ
unittests	66.64% <ø> (-1.09%)	⬇️

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Impacted Files	Coverage Δ
nwbwidgets/ophys.py	67.53% <ø> (-13.09%)	⬇️
nwbwidgets/base.py	68.42% <0.00%> (-3.35%)	⬇️

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