Add BaseImage and ExternalImage

[rly]

Motivation

Fix #2042

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.86%. Comparing base (f956fbb) to head (c803fb9).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           nwb-schema-2.9.0    #2079      +/-   ##
====================================================
+ Coverage             94.83%   94.86%   +0.03%     
====================================================
  Files                    28       28              
  Lines                  2749     2767      +18     
  Branches                708      712       +4     
====================================================
+ Hits                   2607     2625      +18     
  Misses                   82       82              
  Partials                 60       60              

Flag	Coverage Δ
integration	74.34% <100.00%> (+0.16%)	⬆️
unit	84.85% <92.30%> (+0.02%)	⬆️

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[stephprince]

This looks good to me but I think I am not following when something should inherit from BaseImage, Image, or ExternalImage. Should RGBImage, GrayscaleImage and RGBAImage also inherit from BaseImage now so that they can support ExternalImage?