Represents a group in a multidimensional dataset (NetCDF, Zarr, HDF5, and
the rest). Groups hold arrays (GDALMDArray) and subgroups. What a group
holds is a property of it: name and full_name, the path from the root;
mdarray_names and group_names, what is in it; and attributes, the
format's own annotations. Opening one of them by name is open_mdarray()
or open_group().