Allow downloaded CSVs to have exec_time metadata field in seconds

[mattdawkins]

Desktop VIAME produce an exec_time metadata field in their output. This field is in seconds and is only in object trackers not detectors so far. This field could either be read and stored in DIVE and propogated, or it could measure exec_time itself and add it to CSV downloads

[BryonLewis]

Make a note to add this to the Web Version. Pipelines run on the web should store the exec time so that during export it can display that.