fix: DatasetList.get_ids() AttributeError when delayed=True

[Shreyaav18]

Fixes #173

Two small changes to DatasetList in gliderpy/fetchers.py:

1. Initialize self.dataset_ids = None in __init__ for consistency with GliderDataFetcher
2. Fix get_ids() so self.dataset_ids is correctly assigned for both delayed=True and delayed=False

Problem

self.dataset_ids was only assigned inside the if not self.delayed: block. When delayed=True, this block was skipped but the method still attempted to return self.dataset_ids, which was never set, causing:

AttributeError: 'DatasetList' object has no attribute 'dataset_ids'

Changes

__init__: Added explicit initialization:

self.dataset_ids: OptionalList = None

get_ids(): Moved assignment outside the conditional block.

if not self.delayed:
    dataset_ids = [
        dataset_id
        for dataset_id in dataset_ids
        if not dataset_id.endswith("-delayed")
    ]
self.dataset_ids = dataset_ids
return self.dataset_ids

Testing

All 8 existing tests pass. DatasetList currently has no test coverage.