Added
* Added `singlethreaded` kwarg to `Sim` initialization.
* Added `daskkwargs` kwarg to `Sim` initialization.
* Added `percentile` varstat.
Changed
* For all datafiles, switch from `dill` to `cloudpickle` for pickling.
* Parallel processing backend moved from `pathos` to `dask`.
* `Sim.PreProcessCase`, `Sim.RunCase`, and `Sim.PostProcessCase` broken out to
`case_runners.py` functions.
Removed
* Removed `cores` kwarg from `Sim` initialization.
* Removed python 3.7 support, to align with dask.