Write out meta-data JSON sidecars

[adelavega]

Currently, fmriprep does not output .json metadata sidecar files. This makes it cumbersome to get such information (e.g. TR).

For example, @effigies is working on a PR to grabbit/pybids to allow a user to get the metadata of the original bold file that matches a preproc file. This is still not ideal though, because it requires the original dataset. For some applications (e.g. fitlins) that would require having the original dataset around just to get the TR of preproc file.

If fmriprep were to writeout .json sidecar metadata files for its outputs, it would make these datasets much more independent and portable.

[chrisgorgo]

If the only thing you are after is TR you can get it from nifti headers right now. We made sure those are correct.

[effigies]

This might be a good idea also in terms of filtering out the things that can no longer apply after reorientation/resampling, such as PhaseEncoding, SliceTiming, etc.

[adelavega]

Thx @chrisfilo. @effigies we could for now always look in the preproc headers, but I think the issue is that pybids.variables.io uses get_metadata and I suppose looking in the original bold file headers would not be reliable?