Add support for use as part of a wider MPI process

[joezuntz]

The current version of the code always uses MPI_COMM_WORLD as the communicator and calls sys.exit on some processes when the client process exits python.

This change makes it possible to use dask-mpi for part of an MPI process but then return to the original MPI workflow once it is complete. It requires the user to manually call send_close_signal from the client process, and to use a returned value indicating whether the process is the client or not.

[joezuntz]

The test failures shown seem to be part of the original code that is not affected by the changes I've made, and are associated with a timing test. I guess this might be associated with github actions having slowed down since these were written?

[kmpaul]

Some of the errors seen in these tests are related to the bug mentioned in #74.

[kmpaul]

@joezuntz: Can you pull recent updates from the main branch and see what tests pass/fail?

[joezuntz]

I just did this but it seems to show only a single change, to cli.py - is that what you expected?

[joezuntz]

Also it says it's "awaiting approval" now: "First-time contributors need a maintainer to approve running workflows"

[kmpaul]

@joezuntz: Yay! Tests pass, and the failures had nothing to do with your code.

[kmpaul]

@joezuntz: Is this ready to merge, then?

[joezuntz]

Yep, I think good to go - thanks!

[kmpaul]

Thanks @joezuntz!