a command to list DICOM attributes of interest in a pile of DICOMs

[yarikoptic]

attn @loj

quite often it might be very handy to get a quick idea about some metadata within the pile of DICOMs. E.g. for establishing a heuristic or mapping into reproin heuristic like here might be relevant to quickly list values of series_id, protocol_name, image_type. ATM it requires to run -c none to get the dicominfo.tsv dump, and then inspect. That should be produced on drive for each subject(/session). For a quick "glance" I think we need some easier accessible command.

We could extend the list of the ad-hoc commands

  --command {heuristics,heuristic-info,ls,populate-templates,sanitize-jsons,treat-jsons}
                        Custom action to be performed on provided files instead of regular operation.

In principle it should be possible to add a new command but we also already have ls which takes ls function defined in a heuristic. (Only) reproin has one. And I would say that code to support ls might be a bit "overfitting" reproin use case. I think we could pretty much move most of ls code from process_extra_commands into reproin.py:ls and then extend convertall to provide its own desired listing from the kwargs it would get.

• Problem: above commands cannot be passed some other arguments than generic heudiconv arguments (the beast grew organically ;)) ... if outputs in .json format, piping to some json handler (jq) or even just grep might be sufficient for some cases
• thought to (ab)use fscacher here but probably should be done at a different level, not ls specific anyways.

Overall -- not yet sure about the best easiest design to proceed with or what information to output.

[satra]

perhaps this is a better place for something like this: https://github.com/open-dicom/dicom_parser also dicom_parser is where the grouping function of dcmstack is headed, so may be worthwhile thinking of a removal of dcmstack and using dicom_parser.