Adapt unit aware xarray structures [revist in the future]

[euronion]

Working on #194 for CSP lead to #195 as units have always a been intransparent in atlite.

There has been some progress in recent time in implementing unit-aware xarray datastructure, although it is not full ready yet.

We should revisted their progress and think about adapting unit aware datastructures, especially for .convert_and_aggregate(...) with it's different function parameters and return values.

https://pint-xarray.readthedocs.io/en/stable/creation.html

https://github.com/pydata/xarray/issues/525