Add aggregate_time support to convert_and_aggregate

[FabianHofmann]

Closes #489.

Changes proposed in this Pull Request

• add aggregate_time={"sum", "mean", None} to convert_and_aggregate to control temporal aggregation explicitly
• default to aggregate_time="legacy", which preserves historical context-dependent behavior (sum without spatial agg, full timeseries with spatial agg) and emits a FutureWarning nudging users to pick an explicit option
• None means "no temporal aggregation"
• deprecate capacity_factor and capacity_factor_timeseries in favor of aggregate_time
• extract _aggregate_time() helper for consistent temporal aggregation with keep_attrs=True in both code paths
• update wind() and pv() examples to use aggregate_time=None
• add focused tests with shared fixtures for temporal aggregation, deprecated arguments, and invalid argument validation

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.

[brynpickering]

Definitely needed - all my students have been confused by the existing arguments when first working with atlite. My comments are mostly for maintainability but the choice of aggregate_time options for backwards-compatibility might need discussion.

[FabianHofmann]

Thanks for the review @brynpickering! I've addressed all your comments:

• aggregate_time now defaults to "legacy", which preserves the old context-dependent behavior but emits a FutureWarning telling users to switch to an explicit value. Next release we can remove it and default to something explicit (probably "sum"). Replaced False with None.
• extracted the temporal aggregation into a small helper. This also fixes the keep_attrs inconsistency — both "sum" and "mean" now use keep_attrs=True.
• layout and result_ts are now module-level fixtures.

[brynpickering]

This makes more sense to me now. The option one needs to give to ensure backwards-compatibility is much clearer. The next feature release, I would raise an exception on use of capacity_factor and capacity_factor_timeseries but possibly keep legacy, only removing legacy on the subsequent feature release.

Side note: should the use of capacity_factor=True in example notebooks already be converted to using aggregate_time?

[FabianHofmann]

Side note: should the use of capacity_factor=True in example notebooks already be converted to using aggregate_time?

good point, will do that