Update CLI to use use ``post_process`` rather than simplify

[jeromekelleher]

Need to consider the compatibility issues for this.

[hyanwong]

For reference, at the moment we are doing the following in the CLI:

def add_simplify_argument(parser):
    parser.add_argument(
        "--no-simplify",
        action="store_true",
        help="Do not simplify the output tree sequence",
    )

[jeromekelleher]

We can probably map this directly to a new no--postprocess option and say the old --no-simplify is a deprecated alias for this.