Test on GPU in CI

[uellue]

Right now we only cover about half the code in CI since GPU support is missing.

Discussion with @sk1p: Possibly a target for running at JSC? Also allows running the benchmarks

[sk1p]

Discussion with @sk1p: Possibly a target for running at JSC? Also allows running the benchmarks

Good idea! We should reserve a day or so for setting up GPU CI for the various relevant packages (LiberTEM{,holo,blobfinder,live} and sparseconverter?), which is then also a good starting point for continuous benchmarking.