Return target_store from StoreToZarr stage

[jbusecke]

Superseeds jbusecke#1 (see over there for discussion).

[cisaacstern]

This looks good, and the change is from returning nothing (store_dataset_fragment returns None)

pangeo-forge-recipes/pangeo_forge_recipes/writers.py

Lines 69 to 71 in 934e34b

	def store_dataset_fragment(
	item: Tuple[Index, xr.Dataset], target_store: zarr.storage.FSStore
	) -> None:

to returning something, so I don't think we need to worry about a breaking change.

This is simple enough that I think we can leave it without an explicit test. Thanks @jbusecke, great enhancement!

[jbusecke]

Yayyyy! Ill move on with building testing an cataloging stages in the CMIP6 repo. Thanks for the quick merge.