sidecar file for each sample file

[yarikoptic]

target_versions:
  hdmf: 2.1.0
  pynwb: 2.2.0
all_versions:
  ## full list of versions upon inspection of sys.modules

[jwodder]

@yarikoptic I think it might be better (and would definitely be easier) for all_versions to list the versions of all installed packages rather than just the loaded packages. Presumably, we'll be doing the file generation in virtualenvs or other environments that start out minimally-populated, so there shouldn't be much difference between the two sets; moreover, there are issues with reliably getting the package version from a sys.modules entry. This package claims to solve that problem, but it would involve the introduction of more packages into the all_versions list.