Refactor: Centralized HDF5 data loading

[norahaorth]

Changes made:

• Created a combined data loader file (h5_load_data).
• Verified that everything works! I checked the interface, classification scripts, plotting, and Lisa's function.
• Fixed the code in utils.srf_waveforms to make sure the original plot style is kept the same. I tested this by running Lisa's function and plotting single waveforms!
• Deleted the h5_save_data file because it used the old logic?