Zarr bounds should not be discarded, but used to construct Intervals in the dimensions

[felixcremer]

Currently when we read a Zarr dataset we ignore all variables that have "bnd" in their name. Instead of ignoring the bounds we should use this information to construct a dimension with Intervals instead of points.
An example dataset that we can use for that would be at http://tinyurl.com/GLDAS-NOAH025-3H from the geozarr interoperability issue zarr-developers/geozarr-spec#36