Refactor Xarray simulation driver (#124)

* refactor xarray simulation driver

Avoid serialization issues when using dask's distributed scheduler

* black

* add distributed scheduler in tests

* move multi-index logic to stores + clean-up

* update release notes

* move back multi-index logic to driver

Because multi-index levels may be model input variables.

Author: benbovy

doc/whats_new.rst

@@ -6,6 +6,11 @@ Release Notes
 v0.5.0 (Unreleased)
 -------------------
 
+Bug fixes
+~~~~~~~~~
+
+- Fix running batches of simulations using ``dask.distributed`` (:issue:`124`).
+
 v0.4.0 (7 April 2020)
 ---------------------