add parallelization add-nifti-info

[tien-tong]

Closes #478.

Changes proposed in this pull request

• Add parallel processing to cubids add-nifti-info via --n-cpus (defaults to 1).
  • Factor out single-file metadata write into _add_metadata_single_nifti.
• Show a progress bar while add-nifti-info to each json file.
• Allow parallel DataLad saves by adding jobs to CuBIDS.datalad_save(...).
• Add --n-cpus through CLI parsers for:
  • add-nifti-info
  • add-file-collections
  • purge
  • datalad-save

Documentation that should be reviewed

• docs/example.rst: New note and example usage of --n-cpus for cubids add-nifti-info.

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[mattcieslak]

This is probably all good, but we should be sure that if no cpus are requested that it defaults to 1 instead of None. I'm worried None will try to use all the cpus, but I'm not sure that's what actually happens

[mattcieslak]

thanks!!