Do more to check for unexpected mpiexec

[eirrgang]

There are various ways a user could accidentally run a script in several processes, such as with mpiexec and for gmxapi not to notice (e.g. if mpi4py is not installed or is not triggered because the work graph does not require parallelism). There should be some heuristics and precautions we can apply to minimize the chance of running multiple copies of the same work graph in the same directory at the same time, such as checking for environment variables and using lock files.

For convenience, if a lock file doesn't get cleaned up from a failed job, we can add a force_remove_lock=true option to run()