update infrastructure to support tethered simulations

There are some common CTSM simulations that require a predefined sequence of tethered segments that depend on each other. One example is running the AD/SASU/postSASU sequence to spinup stability. Another example is running a transient historical case (e.g. TRENDY), where streamfiles and namelists are edited when the simulation reaches a critical year. It would be nice to have the sequence proceed automatically, instead of having to babysit the simulations, waiting for each segment to finish and then manually running a shell script to advance to the next segment. I'm working on a PR to provide a first draft of some software that could potentially automate this using the PBS dependency flags.