Soft deprecate `sched.test_cmd`

[Paul-Ferrell]

The name of sched.test_cmd has bothered me for a long time. It's time to change it.

Rename to sched.mpi_cmd and make sched.test_cmd an alias to that.

Update documentation and examples to use the new name.

[Paul-Ferrell]

@sly likes sched.mpi_launch better. @sly has good taste.

Do that.

[Paul-Ferrell]

Better yet, sched.launch.