feat: add optional redshift to PointDataset

[Jammy2211]

Summary

Add optional redshift to PointDataset with CSV round-trip support so cluster-scale strong-lens workflows can encode per-source redshifts in a single hand-editable CSV alongside positions, fluxes, and time delays. Existing point-source use cases are unaffected — the parameter defaults to None and the CSV column is only emitted when at least one dataset sets it.

This is a library prerequisite for the cluster-scale simulator being added in autolens_workspace/scripts/cluster/simulator.py (follow-up PR on autolens_workspace).

Closes #464 (partial — workspace PR follows).

API Changes

• PointDataset.__init__ gains optional redshift: Optional[float] = None kwarg; new redshift attribute on the instance (float or None).
• CSV I/O (output_to_csv, list_from_csv, to_csv, from_csv) now round-trips a redshift column, emitted when any dataset supplies one.
• Validation: a name group in a loaded CSV must share a single redshift across all rows — mixed or partially populated values raise ValueError.

See full details below.

Test Plan

• python -m pytest test_autolens/point/test_dataset.py — 10 passed (6 pre-existing + 4 new)
• python -m pytest test_autolens/point/ — 48 passed (full point-source suite)

Full API Changes (for automation & release notes)

Added

• PointDataset.__init__(..., redshift: Optional[float] = None) — new optional keyword argument; stored on the instance as self.redshift.
• PointDataset.redshift: Optional[float] — new instance attribute.
• CSV column redshift — emitted by output_to_csv/PointDataset.to_csv when any dataset in the input has a non-None redshift; consumed by list_from_csv/PointDataset.from_csv.
• Module-level _REDSHIFT_HEADERS = ["redshift"] constant.
• Module-level _group_redshift(group_rows, group_name) helper.

Changed Behaviour

• list_from_csv: when the CSV contains a redshift column, every row in a given name group must share the same redshift value; partial population or mixed values raise ValueError.
• PointDataset.info: output now includes a redshift : line.

Migration

• No migration required. Existing callers and CSV files continue to work unchanged; the redshift column is only added when a caller opts in.

🤖 Generated with Claude Code

[Jammy2211]

Workspace PR: PyAutoLabs/autolens_workspace#77