`Units.waveform_unit` parameter has no effect on serialized `waveform_mean` unit attribute

[h-mayorquin]

The Units class accepts a waveform_unit parameter (pynwb misc.py L174-175) and pynwb stores it correctly on the container, but the value is never propagated to the HDF5 file. The NWB schema defines the unit attribute on waveform_mean with a fixed value (value: volts) in nwb-schema nwb.misc.yaml L259-262, and hdmf enforces this by short-circuiting attribute resolution when spec.value is set in hdmf objectmapper.py L1061-1062. This means the ObjectMapper code in pynwb that maps waveform_unit to the unit attribute is never reached, making the waveform_unit parameter effectively dead code.

My personal view is that the unit attribute should either be made configurable by changing value: volts to default: volts in the schema, or the fixed value should be changed to microvolts which is the natural unit for extracellular electrophysiology recordings.