Ignore all subjects except the requested ones in BIDSLayout initialization

[tsalo]

What would you like to see added in fMRIPrep?

I believe the BIDSLayout initialization might take less time on large datasets if the ignore patterns included a regular expression to select any participant folders except the ones requested by the user.

ignore_patterns = [
    'code',
    'stimuli',
    'sourcedata',
    'models',
    re.compile(r'^\.'),
    re.compile(
        r'sub-[a-zA-Z0-9]+(/ses-[a-zA-Z0-9]+)?/(beh|dwi|eeg|ieeg|meg|perf)'
    ),
]
if cls.participant_label:
    # Ignore any subjects who aren't the requested ones.
    ignore_patterns.append(re.compile(r'sub-(?!' + '|'.join(cls.participant_label) + r')\w+'))

_indexer = BIDSLayoutIndexer(
    validate=False,
    ignore=ignore_patterns
)

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

This stems from a similar request in QSIPrep (PennLINC/qsiprep#513).