tune _parse_scan_metadata to prefer 'duration' field from _scans.tsv when present

[yarikoptic]

WiP going to equip BIDS

• [ENH] Add optional 'duration' column to _scans.tsv bids-standard/bids-specification#2508 and from there on in -examples and -validator

and heudiconv to populate it:

• Add acquisition duration tracking to BIDS _scans.tsv files nipy/heudiconv#872

Ideally we should have

Examples could be found e.g. in bids-examples, but I will just provide that information soon in our dbic/QA (WiP to populate)

With that in mind, all custom functionality should be abandoned so we do not have burden to maintain that DICOM/neuroimaging specific code here -- we might want to reuse that logic if possible (later) from heudiconv.