In particular this ensures we don't circumvent the error checking of the
final process exit status when transitioning from Running to Zombie.

This allows the user to specify the expected state of a process at the
end of a simulation.

This also changes the default behavior to treat a process still running
at the end of the simulation as an error.

See shadow#2034

Explicit yaml types can be difficult to work with. e.g. it's unclear how
to get pyyaml to emit `!running` (and not `"running"` or `!running {}`.
Even if we solved it for pyyaml, users are likely to run into problems
with other tools.

With a few tweaks we get a more intuitive yaml representation, at the
expense of a slightly more complex internal representation.