ecg_intervalrelated() output formatting

[Mitchellb16]

Just wondering if anyone could explain the rationale for the current formatting output by ecg_intervalrelated, being the HRV features (columns) are series of arrays of arrays of single floats.

This prevents some dataframe functionality, e.g. quick plotting with df.HRV_MeanNN.plot(), but I wanted to know if there are other benefits to it that I'm not seeing. Thanks!