fix: resolve xarray MultiIndex FutureWarning in build_renewable_profiles

[FabianHofmann]

Changes proposed in this Pull Request

Fixes the FutureWarning from xarray about pandas MultiIndex when calling atlite's conversion functions in build_renewable_profiles.py:

FutureWarning: the `pandas.MultiIndex` object(s) passed as 'bus_bin' coordinate(s) 
will no longer be implicitly promoted and wrapped into multiple indexed coordinates...

• Pass a RangeIndex to atlite instead of the bus_bin MultiIndex, then explicitly assign the MultiIndex coordinates on the result using xr.Coordinates.from_pandas_multiindex() before unstacking.
• Remove redundant log messages that duplicate the timing-based info logs already present.

Note that atlite currently fails when directly passing the xr.Coordinate as it expects the index object to have a .name attribute. so this change is save for future xarray version, but we need to support xr.Coordinates in atlite in the first place.

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.rst.

If applicable:

• N/A