Add PTB-XL ECG dataset support

[sajus2023]

Add PTBXLDataset class for loading the PTB-XL electrocardiography dataset, a large publicly available dataset containing 21,837 clinical 12-lead ECG records from 18,885 patients. Current CardiologyDataset does not support PTB-XL 1.0.3

Features:

• Support for both 100Hz and 500Hz sampling rates
• Automatic metadata preparation from raw PTB-XL files
• Diagnostic superclass label extraction from SCP codes
• Integration with PyHealth's BaseDataset architecture