Add waveforms to units table

[CodyCBakerPhD]

Motivation

Upstream changes to hdmf now allow us to add jagged waveform data as columns to the unit table. This necessarily accompanies sampling information about the waveform data, such as sampling rate and conversion factor for casting data types that are not in Volts.

How to test the behavior?

spike_times = [1, 2]
wf_data = [[1,2,3], [1,2,3]]  # for instance
nwbfile.add_unit(id=1, spike_times=spike_times, waveforms=wf_data)

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• [NA] Have you included the relevant issue number using #XXX notation where XXX is the issue number?

@bendichter Opening this as a draft to start - was there anything else we wanted added to this PR?

[rly]

Looks good to me! Thanks.

[rly]

The failing miniconda36 test is due to a CI configuration issue that will be resolved separately.