Support updated Images, ImageReferences, IndexSeries types

[rly]

Motivation

NWB 2.5 will introduce changes to Images and IndexSeries and it introduces a new neurodata type ImageReferences.
See https://github.com/NeurodataWithoutBorders/nwb-schema/blob/dev/docs/format/source/format_release_notes.rst

This PR adds support for these schema changes.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[oruebel]

Looks like the flake8 test is failing, but otherwise this looks good

[codecov]

Codecov Report

Merging #1483 (e420e62) into dev (e05b553) will decrease coverage by 0.04%.
The diff coverage is 73.52%.

@@            Coverage Diff             @@
##              dev    #1483      +/-   ##
==========================================
- Coverage   78.28%   78.24%   -0.05%     
==========================================
  Files          37       37              
  Lines        2772     2799      +27     
  Branches      465      473       +8     
==========================================
+ Hits         2170     2190      +20     
- Misses        522      524       +2     
- Partials       80       85       +5     

Impacted Files	Coverage Δ
src/pynwb/io/core.py	61.11% <ø> (-0.30%)	⬇️
src/pynwb/image.py	86.73% <52.63%> (-8.33%)	⬇️
src/pynwb/base.py	95.23% <100.00%> (+0.26%)	⬆️
src/pynwb/io/file.py	82.38% <100.00%> (+0.11%)	⬆️
src/pynwb/core.py	65.85% <0.00%> (+1.21%)	⬆️

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[oruebel]

Looks good