outsource cordex.cmor to standalone package

[larsbuntemeyer]

There is lots of code that just deals with managing cmor tables, meta info, time axis, bounds.... Maybe we can make that more generally useful in an external package and maybe implement a full xarray backend for writing in addition to the cmor library.

• Be very flexible in output format, e.g., write directly to zarr, object stores, etc..
• Create cmorized datasets lazily (without having to write NetCDF files)...