This merge request includes the following changes:
fixed a bug in the group_detections function in the detection module. The bug was causing singular detections to be dropped if they occurred at the end of a batch.
added a merge argument to the process functions in the detection module. With merge=True, the function merge_overlapping_detections is applied to the detections before they are returned. The default value is merge=False to ensure backward-compatability.
added methods for getting file paths and file durations in the AudioFrameLoader and FrameStepper classes.
added function in selection_table module for computing aggregate duration of overlapping annotations
