[note] [dev] Removal of set_container_sources() method in IPFX 2.0.0

[sheriferson]

This is a note for future development.

While upgrading PyNWB and supported Python versions in version 2.0.0 of IPFX in #559, we removed set_container_sources() (see c766514 for more context/background).

Testing hasn't revealed any issues arising from the removal of the method and its use, but this issue is here as a note for the near future: should we run into issues validating NWB files for publishing, the commit linked above might be a good place to look for possible root causes.