Add better support for multi-product archives

[pont-us]

An EnMAP .tar.gz archive contains one or more .ZIP sub-archives, each containing one EnMAP product. Currently, when opening a multi-product archive or directory, xarray-enmap simply opens the first product and ignores the rest. We should consider better ways to handle this.

Adding a lazy open_datatree implementation to EnmapEntrypoint might be the most appropriate solution.

Additionally or alternatively, we could add a filter parameter to the open_dataset implementation to allow the caller to select a known, specific sub-archive, e.g. by name or substring thereof.