Following on from the discussion #295 about mail features it should be possible to add some minimal support for mailing users.

While it is possible for a user to add specific flags to qsub_flags there are a couple of extra cases that would be good to cover:

1. Mail when a series of runs has finished, i.e. when payu is invoked like payu run -n N and the Nth run completes
2. Mail when the job fails. This is currently not supported in PBS, though the documentation suggests it does have this feature, the error condition it references is an internal PBS state, and not the exit code of the job