fix: use xarray.Dataset copy instead of constructor

[maurerle]

minimal non-working example:

import pypsa
network = pypsa.examples.ac_dc_meshed()
network.optimize()

Changes proposed in this Pull Request

since the latest xarray version, passing a Dataset as data_vars to the Dataset constructor is not supported. Use ds.copy() to create a copy of a Dataset.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.