peri-event HDF5 output should include trial information

[celefthe]

`process peri-event` HDF5 output carries only `/trial_index`, `/event_time` and a `trials_path` attr; the trials columns are not in the file like they are for the CSV output #158.

Write a `/trials` group with one dataset per trials CSV column, kept rows only, in `/trial_index` order, strings as `S` dtype.