v0.97gpu

What's Changed

• freeze_out_surface = 0: stop without building the freeze-out surface by @jhputschke in #9
• Source term on the GPU path: skip empty fills, bin strings by transverse reach by @jhputschke in #10
• StringFind4: fail loudly on a parameter file without EndOfData by @jhputschke in #11
• Freeze-out surface: parallel over eta slices again, deterministic; store P by @jhputschke in #12

Full Changelog: v0.96gpu...v0.97gpu