reduceArc should take a set of arcs and produce a pfsArm file

The arcs should be raw, so this ticket will need to run IsrTask as part of its work.

This ticket should remove registering the master arc, but not register the pfsArm file (see PIPE2D-215). The master (high-SN) arc need never be written to disk, unless there is a reason to write it for debugging.