Waveform conversion factor for Units table column

[CodyCBakerPhD]

This topic is very analogous to an older discussion, Issue #273, which culminated in the addition of basic waveform properties of waveform_unit and waveform_rate to the columns of the simple statistics waveform_mean and waveform_sd, which were all that existed at the time. In that discussion, it was a fair point for these simple, scalar statistics to be fixed to units of Volts and remain unscaled by any factor.

With schema Issue #248 and pynwb PR #1330, the Units table was extended to include an entire jagged array as a column for the actual waveform data.

As such, I was hoping we could revisit the topic of including a conversion factor for that column, since right now we would have to write all the data in float (converted) form whereas it is common for it to exist in a more base (usually int16) format with an attached conversion factor, as per the ElectricalSeries schema. For long recordings (that are either included or not included in the same NWBFile), this would save a fair amount of storage space even without using compression (though that is also expected to improve if int data types would be allowed).

cc: @bendichter

[rly]

Good idea. This sounds good to me.