Fix add_data_interface functionality

[stephprince]

Motivation

Fix #2050.

In #2011 I mistakenly removed the functionality of add_data_interface when updating the deprecation warning. I think add_data_interface should still operate as expected until the function is fully removed and replaced by add.

How to test the behavior?

from pynwb.testing.mock.file import mock_NWBFile
from pynwb.testing.mock.ecephys import mock_ElectricalSeries


nwbfile = mock_NWBFile()

# Create processing modules
ecephys_module = nwbfile.create_processing_module('ecephys', 'Example ecephys data')

electrical_series = mock_ElectricalSeries()
ecephys_module.add_data_interface(electrical_series)

assert len(nwbfile.processing["ecephys"].data_interfaces) == 1

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.85%. Comparing base (01d9ab9) to head (d3c662c).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2052   +/-   ##
=======================================
  Coverage   92.84%   92.85%           
=======================================
  Files          28       28           
  Lines        2755     2756    +1     
  Branches      716      716           
=======================================
+ Hits         2558     2559    +1     
  Misses        127      127           
  Partials       70       70           

Flag	Coverage Δ
integration	73.69% <0.00%> (-0.03%)	⬇️
unit	82.76% <100.00%> (+<0.01%)	⬆️

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