Add support for external store of `LFP` and `WaveformSet` data

[kabilar]

• LFP.Electrode::lfp
• WaveformSet.Waveform::waveforms
• We need to decide if external store will be the only method for storing these attributes or if a configuration would allow for selecting external or internal store.

[kabilar]

Would suggest that users who would like to use external store to fork the repo and make the changes to the respective table definitions. For reference, see the BrainCOGS fork (LFP.Electrode table, WaveformSet.Waveform table).