Allow launching simulations in a different environment

[AngelFP]

Some simulation codes might have to be installed (and therefore executed) in a different environment than optimas (e.g., due to different dependencies, MPI version, etc). This PR allows the TemplateEvaluator to launch simulations in their own environments if an env_script is provided (a new feature in libEnsemble 0.10). This is a shell script that loads the required environment.

If the environment of the simulation uses a different MPI than optimas, this should be specified by indicating a env_mpi.

As an example, this would be the template evaluator for running simulations with a version of HiPACE that requires a different environment and which uses openpmi.

ev = TemplateEvaluator(
    sim_template='template_simulation_script',
    analysis_func=analyze_simulation,
    executable='/path/to/build/bin/hipace',
    n_gpus=2,
    env_script='/path/to/profile.hipace',
    env_mpi='openmpi'
)

The PR also adds a HiPACE example that exposes these options if needed.