Update _timeseries.tsv time activity curves outputs to _tacs.tsv

[mnoergaard]

Currently, the time activity curves with timing information are outputted to a _timeseries.tsv file similar to the confounds file. However, while similar in nature, we propose (following the PET-BIDS specification), that time activity curve files instead have the _tacs.tsv filename with the_tacs suffix.

[mathesong]

Agree strongly on this point. For quantification at ROI level (which is most common in PET), the TACs are the primary output from image analysis. So if not _tacs.tsv, they must be clearly identifiable for a subsequent quantification app as being the file containing the TACs as distinguishable from any other timeseries such as movement artefacts etc. Another possibility is having another required attribute to state what it is, as well as a set of defined outputs, one of which is for TACs. I think the suffix _tacs makes most sense given its importance and given its presence in virtually every PET derivative output from image analysis.

[mathesong]

I've made a PR to this effect here. Do let me know if I've missed any important instances.

[mnoergaard]

Closed as completed via #114