Allow timezone-naive timestamps reference time on read

[stephprince]

Motivation

Fix #2054. Allow files with a timestamps reference time that is not timezone-aware to be read.

How to test the behavior?

In MATLAB:

nwb = NwbFile( ...
    'session_description', 'Simple demo file', ...
    'identifier', 'test_file', ...
    'session_start_time', datetime("now"), ...
    'timestamps_reference_time', datetime("now"));
nwbExport(nwb, 'temp.nwb')

In python:

from pynwb import NWBHDF5IO

io = NWBHDF5IO('temp.nwb', mode="r")
nwbfile = io.read()

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 ruff check . && codespell 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.

[stephprince]

Note that timezone information is automatically added to session_start_time if it is missing. Instead of throwing an error, would it be better to also automatically add timezone information to timestamps_reference_time if missing?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.85%. Comparing base (2888ad0) to head (9f03c11).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2056   +/-   ##
=======================================
  Coverage   92.85%   92.85%           
=======================================
  Files          28       28           
  Lines        2756     2756           
  Branches      716      716           
=======================================
  Hits         2559     2559           
  Misses        127      127           
  Partials       70       70           

Flag	Coverage Δ
integration	73.69% <0.00%> (ø)
unit	82.76% <100.00%> (ø)

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