Make band_mean and band_sd in DecompositionSeries optional

[rly]

The docs for band_mean and band_sd in DecompositionSeries say that these fields are specific to Gaussian filters. Not all filters used in neurophysiology are Gaussian though, e.g., an LFP decomposition into alpha, beta, and gamma bands is not Gaussian, and LFP decomposition is supposed to be supported by this neurodata type. These table columns should be optional.

Relatedly, the DecompositionSeries.bands table should be its own type. See #336