Merlin

Fixed
- Bug that caused step restarts to lose alternate shell specification, and
associated CLI `restart_shell` test.

Fixed
- Bug that caused example workflows with a variable reference in their
name to be listed by `merlin example list` with variable reference notation.
- Bug that caused `requirements.txt` files to be excluded from generated
`merlin example` dirs.
- Bug that causes step restarts to lose alternate shell specification. Also added
CLI test for this case.

Changed
- Default broker server password is now `jackalope-password`, since `rabbit` is
currently accessed by developers only.

Added
- The first version of an optimization workflow, which can be accessed with
`merlin example optimization`.
- Dev requirement library for finding dependencies (and `make reqlist` target)

Changed
- Improved warning and help messages about `no_errors`

Fixed
- Pinned to celery>5.0.3 so that `merlin purge` works again

Changed
- Now requiring Celery version 5.x.
- Further improvements to the `null_spec` example.

Fixed
- Users will no longer see the message, "Cannot set the submission time of '<step name>'
because it has already been set", when tasks are restarted.
- Bug causing `merlin restart` to break.

Changed
- Improved internal logic beyond the crude fixes of the prior 2 patches.
- Added a developer cli test for the minimum valid spec format.
- Improvements to the `null_spec` example, used for measuring overhead in merlin. Includes
a new `null_chain` and removes the now-redundant `sim_spec`.

Fixed
- Completed 1.7.2 fix for `merlin run-workers`.