Storm uncertainty

[JosephThomasParker]

This adds workflows for @johnomotani 's Storm2d turbulence example, https://github.com/johnomotani/2d_turbulence_example

There are two workflows:

1. workflows/storm/pce/example_pce which runs UQ with third-order polynomial chaos, varying the first four parameters John suggested. This requires 256 simulations...
2. workflows/storm/sc/example_sc which runs UQ with adaptive stochastic collocation, also varying those four parameters. This is an iterative method, and is currently set to exit after 20 iterations (about 130 simulations).

This is a WIP because the Storm2d model isn't properly an external of this repo - I just moved the data dir and storm2d executable into models/storm. I don't know how to set this up properly (and we need to be a bit careful, as this public version of Storm requires BOUT v4.3.0).

These scripts also don't currently support batch submission, or restartable campaigns. We (i.e. Peter) did have those features working before, but we need to update them for the most recent EasyVVUQ API. That said, these scripts should be okay to run on clusters as a single job invoked as python example_*.py, so long as uq.actions.local_execute is updated to be an mpirun command. The SC case would be better to try this with, as it produces meaningful output as it goes along.