Adopt BIDS derivative standard for disc labels

[jcohenadad]

When running manual_correction.py following the wiki tutorial, the created labeled had the suffix _label-disc.

It should instead be: _label-discs_dlabel as per our internal doc

[valosekj]

Good point! Indeed, the code has not yet been updated according to the latest convention.

Related issue: #77

Assigning @NathanMolinier