idea: heuristic call back after converting a subject/session or dcm2niix conversion

[yarikoptic]

somewhat of an alternative to #691 in the use case from @michellewang et al:

• rename some files base on dcm2niix extracted and harmonized metadata to add _dir- field for fieldmaps
• might be most efficient if was done right after dcm2niix conversion, and result return would provide "adjusted filenames" or may be even "filenames renaming mapping" so heudiconv could take care about all necessary renames and adjustments, and then pass it to populate _scans with the adjusted filenames