Add task for long-term archiving

[forsyth2]

Add task for long-term archiving. This may be outside the scope of zppy, being archiving rather than post-processing. However, it seems possible and useful to have zppy handle the initial steps of long-term archiving: cleaning up the directory and transferring the simulation data to HPSS. (Once E3SM-Project/zstash#154 merges, this will be relatively simple).

Note that the zstash check step would still have to be done on the machine the data was transferred to.