Delineate processed shot .npz files in same signal group folder by machine name

Currently, preprocessing dumps shots from all machines/shotlists with the same
signal group hash into the same folder as `.npz` files. There were no collisions in file
names because the D3D (125500 - 168555) and JET (66027 -  92504) shot numbers that we have used do not currently overlap.

For future proofing, we should prefix these files with `jet_` or `d3d_`, etc.