Add waveform conversion factor and offset to schema

[CodyCBakerPhD]

Summary of changes

Proposing the addition of conversion factors and offsets to waveform columns of a Units table.

For large amounts of recording data, even adding waveform snippets to the columns of a Units table can be a significant task. One step towards reducing unnecessary data inflation is to add attributes for scaling factors similar to the behavior of other TimeSeries-like objects that allow the data to be stored in some minimal base type, while the conversion factor then scales it into the specified scientific units.

I've added these attributes and attempted to propagate them through the IO, mirroring the patterns established by the waveform_rate attribute.

Sister PR: NeurodataWithoutBorders/pynwb#1422

PR checklist for schema changes

To-Do:

• Update the version string in docs/format/source/conf.py, core/nwb.namespace.yaml, and /core/nwb.file.yaml
  to the next version with the suffix "-alpha"
• Add a new section in the release notes for the new version with the date "Upcoming"
• Add release notes for the PR to docs/format/source/format_release_notes.rst