Allow fractional seconds in scans file datetimes

[tsalo]

This stems from bids-standard/bids-specification#470, and is related to nipy/heudiconv#447 and nipy/heudiconv#451.

In bids-standard/bids-specification#470, I am proposing that fractional seconds be allowed in the accepted datetime format for the acq_time column in scans files. @sappelhoff pointed out that the validator will need to be updated in line with this change to the specification. The locations that need to be updated in the validator are below:

https://github.com/bids-standard/bids-validator/blob/ecacf0d30f9bc803393c1bb9833179d47a5d5451/bids-validator/validators/tsv/checkAcqTimeFormat.js#L4-L12

https://github.com/bids-standard/bids-validator/blob/920d1957c1788c96f3df5b0be3eb7c2be594f8d3/bids-validator/utils/issues/list.js#L465-L470

I can update list.js easily enough, but I don't know Javascript, so I'm not sure how to update checkAcqTimeFormat.js. Any help anyone can give would be appreciated.

[sappelhoff]

cc @rwblair

[rwblair]

Fortunately looks like the library we use supports fractional seconds:
https://date-fns.org/v2.9.0/docs/parse

I'll try updating the format string to use them and seeing how it plays with existing datasets.

[tsalo]

The associated bids-specification PR was merged, so I just wanted to ping this issue in case anyone was waiting for fractional seconds to be officially supported.