build_dsp() should write a Struct of Tables

[gipert]

Meaning that the DSP tables for each channel would sit in a lgdo.Struct, instead of sitting in a bare HDF5 group.

This would be useful to allow users to read the entire /dsp in memory at once (or use the field_mask argument to apply a filter). Given a list of channel, one can only do so by manually looping, at the moment.