Add aggregate_time support to convert_and_aggregate

[FabianHofmann]

Closes #489.

Changes proposed in this Pull Request

• add aggregate_time={"sum", "mean", False} to convert_and_aggregate to control temporal aggregation explicitly
• preserve historical default behavior via aggregate_time=None
  • without spatial aggregation: sum over time
  • with spatial aggregation: return full time series
• deprecate capacity_factor and capacity_factor_timeseries in favor of aggregate_time
• apply temporal aggregation consistently for both spatially aggregated and non-aggregated outputs
• update wind() and pv() examples to use aggregate_time=False
• add focused tests for temporal aggregation, deprecated argument handling, and invalid argument validation
• add a release note entry for the upcoming release

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• 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.