GitLab

This merge request includes a new GitLab CI YAML file that allows us to trigger CI pipelines for Darshan on Theta at the ALCF.

Build tests are automatically scheduled on push to master, as well as on manual trigger from web or from schedules. Run tests (i.e., regression testing via Theta batch scheduler) can only be scheduled via web interface or via schedules (and must additionally pass RUN_TESTS=true as a variable).