Issues opening certain stores

[jbusecke]

Many thanks to @Yadidya5 for bringing an issue with a certain store to my attention here.

We concluded over there that the problem is likely related to the zarr store itself (since we cannot even open it properly), and thus here is the better place to keep track of that.

I propose that any refactor as in #31 should probably have some rudimentary test of each store, ensuring that it can be opened with e.g. xr.open_zarr(store).