Create a new output variable, potentially over-writing existing or creating new.

def _create_variable(orig_v, step, variables):
    """Create a new output variable, potentially over-writing existing or creating new.
    """
    # get current variable, and convert to be the output of our process step
    try:
        v = _get_variable(orig_v["id"], variables)
    except ValueError:
        v = copy.deepcopy(orig_v)
        if not isinstance(v["id"], six.string_types):
            v["id"] = _get_string_vid(v["id"])
    for key, val in orig_v.items():
        if key not in ["id", "type"]:
            v[key] = val
    if orig_v.get("type") != "null":
        v["type"] = orig_v["type"]
    v["id"] = "%s/%s" % (step.name, get_base_id(v["id"]))
    return v