Refactor our "--command ls" into an external script to extract DICOM metadata

[yarikoptic]

ATM --command ls output is very very limited -- outputs only information about study (locator, subject, session) and used to sort through DICOMs in a cron job, quite inefficiently.

It would be great to have more exhaustive and efficient regular extraction/update of information about DICOMs on the drive, which would be used later not only by heudiconv but also by reprostim for synchronization purposes.

TODO 4 @yarikoptic : detail it up

Attn @vmdocua -- please review code in https://github.com/nipy/heudiconv/blob/master/heudiconv/dicoms.py#L67 on what metadata we extract from DICOMs. Further details on our setup at DBIC could be found in https://github.com/repronim/reproin?tab=readme-ov-file#getting-started-from-scratch