Discussion on execution environments for PyARC codes

This comment on !176 (merged) raises a topic on whether or not PyARC expects the execution environment of all codes to be the same or not. Expecting them to be the same greatly simplifies expectations about executing these codes, particularly executing them in parallel w/ MPI. But given that we're working with a combination of older codes (i.e. the ARC suite), which may be more restrictive in terms of compiler/library versions, and and newer codes (i.e. DASSH & OpenMC), it may be beneficial to support different execution environments.

Maybe @nstauff can comment on some of the requirements (or lack thereof) for the ARC codes? I don't understand them fully at this time.