add UnitSeries

[bendichter]

Motivation

Part of plan to make spike waveforms accessible from Units table. See https://github.com/NeurodataWithoutBorders/nwb-schema/issues/248

How to test the behavior?

from pynwb.misc import Units, UnitSeries

units = Units()
us = UnitSeries('test_unit_series', np.array([0, 0, 1, 1], dtype=int), timestamps=[.1, .2, .3, .4], units=units, description='description')

Checklist

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