add base class to all data units

[aler9]

needed by #2068

[codecov]

Codecov Report

Merging #2122 (34d6cb4) into main (db3862c) will decrease coverage by 0.14%.
The diff coverage is 6.31%.

@@            Coverage Diff             @@
##             main    #2122      +/-   ##
==========================================
- Coverage   62.27%   62.14%   -0.14%     
==========================================
  Files         107      108       +1     
  Lines       11357    11375      +18     
==========================================
- Hits         7073     7069       -4     
- Misses       3773     3794      +21     
- Partials      511      512       +1     

Files Changed	Coverage Δ
internal/core/rpicamera_source.go	0.00% <0.00%> (ø)
internal/core/udp_source.go	0.00% <0.00%> (ø)
internal/formatprocessor/av1.go	48.10% <0.00%> (+1.18%)	⬆️
internal/formatprocessor/generic.go	50.00% <0.00%> (+3.33%)	⬆️
internal/formatprocessor/h264.go	74.66% <0.00%> (+0.66%)	⬆️
internal/formatprocessor/h265.go	73.64% <0.00%> (+0.61%)	⬆️
internal/formatprocessor/mpeg2audio.go	0.00% <0.00%> (ø)
internal/formatprocessor/mpeg4audio_generic.go	0.00% <0.00%> (ø)
internal/formatprocessor/mpeg4audio_latm.go	0.00% <0.00%> (ø)
internal/formatprocessor/opus.go	0.00% <0.00%> (ø)
... and 5 more

... and 2 files with indirect coverage changes

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