Change statefulCompute/statelessCompute that access runDir

[robnagler]

Code like this should not be written:

def stateless_compute_read_epics_values(data, **kwargs):
    _PREV_EPICS_FILE = "prev-status.json"
    # TODO(pjm): hacked in animation directory
    run_dir = pkio.py_path(
        re.sub(r"/unused$", "/animation", str(simulation_db.simulation_run_dir(data)))
    )
    p = run_dir.join(_PREV_EPICS_FILE)

This is an analysisJob, not a statelessCompute.

BTW, globals (_PREV_EPICS_FILE) should be defined globally (with global), but in this case, it can be local to make the code clearer:

    run_dir.join("prev-status.json")

Also,

    shutil.copyfile(str(e), str(p))

This is not necessary. The reason we chose py.path is that it solves this problem for us:

    e.copy(p)

And this should be encapsulated in pkio:

        and e.exists()
        and p.exists()
        and filecmp.cmp(str(e), str(p), False)

See #6289 for why.