option to add event markers when plotting BehavioralEvents data

[stephprince]

The BehavioralEvents data interface seems to be used for time series with irregular events (https://nwb-schema.readthedocs.io/en/latest/format.html?highlight=events#behavioralepochs). In the example file below, all the data have the same value but the timestamps are irregularly spaced. Adding the option to plot markers+lines could help with visualization (like shown in the nwbwidgets display in the bottom of the picture below)

The example is from acquisition.lick_times.time_series.lick_left_times from this file from dandiset 00006