interoperability fixed. We were given a Zarr format dataset
stored as a directory+file tree. This dataset uses the XArray
conventions and was generated by some non-Unidata Zarr implementation.
In attempting to process it with NCZarr, several interoperability
problems were discovered and fixed. This gives us more confidence
that NCZarr -- using pure zarr -- can interoperate with other
Zarr implementations.
Specific changes:
* Add test nczarr_test/run_interop.sh
* Support attributes with single value not enclosed in JSON array tags.
* Add mode inferencing and use it in nczarr_test/run_purezarr.sh
* Reduce size of tst_err_enddef.nc because it is more than 3 GB.
