Retain sub-second resolution in acq_time column of scans tsv files

[tsalo]

Summary

I am working on a method to synchronize physio data from either BioPac or Labchart with BIDS-format scan data in order to convert the physio data to BIDS format (see physiopy/phys2bids#219). This requires sub-second acquisition timing information from the scan data. At the moment, I use the AcquisitionTime field heudiconv retains in the json files, but I think that the scans.tsv file would be easier to access. The only unfortunate thing is that, in the conversion, sub-second timing information is discarded. See below:

heudiconv/heudiconv/bids.py

Lines 407 to 411 in 5681e0d

	try:
	date = dcm_data.ContentDate
	time = dcm_data.ContentTime.split('.')[0]
	td = time + date
	acq_time = datetime.strptime(td, '%H%M%S%Y%m%d').isoformat()

If that information could be retained, it would make it easier to synchronize the scans with externally-acquired data, including physio.

[yarikoptic]

indeed, I see no immediate reason why we did not store sub-second resolution. Probably with multi-second TRs etc we simply did not care enough. heh heh