GPU Continuous Integration (CI) Testing

[jtramm]

OpenMC currently uses the GitHub Actions platform for continuous integration testing of all new features, which are proposed as GitHub pull requests. GitHub Actions offers hosted virtual machines to run workflows, but none of the GitHub-hosted runners have GPUs available.

For this task, we will establish CI testing specifically for a variety of GPU architectures to ensure that GPU support is properly maintained over time. As with the CI testing for benchmarks described above, we will be able to leverage the JLSE cluster at ANL.