Add filter labelling

[alexhroom]

This PR adds BatchData.set_label() which allows you to set a label for a filter set and index the filter set using its label. Fixes #111.

[Copilot]

🟡 Changes recommended

The missing-label error behavior conflicts with its documentation, and related documentation corrections remain needed.

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Pull request overview

Adds BatchData.set_label() and label-based filter-set indexing.

Changes:

• Adds label assignment, lookup, validation, and lifecycle handling.
• Supports labels in filter-set operations.
• Documents label usage and combination behavior.

File summaries

File	Summary	Review findings
src/batch/interface.rs	Implements filter-set labels and label resolution.	Update get_histogram and get_n_events docstrings to support int | str; fix the missing-label error type or remove the KeyError claim.
docs/source/tutorials/batch_processing.ipynb	Documents labeling and batch operations.	Clarify that + retains histogram settings while * creates fresh histograms.

Review details

Suppressed comments (2)

docs/source/tutorials/batch_processing.ipynb:401

• + does not reset histogram settings: add() clones the left operand's results, retaining its time range and bin count, whereas * constructs fresh histograms. This statement therefore misdocuments the behavior; distinguish the label loss from the different histogram-setting behavior of + and *.

    "Note that as `+` and `*` create new filter sets from old ones, any existing histogram settings or labels are lost when using those operations."

src/batch/interface.rs:709

• Error::msg(...) creates an anyhow message, not a PyErr, so PyO3's anyhow conversion cannot downcast it to KeyError; a missing label will be exposed as a RuntimeError, contrary to this comment. Either propagate an actual PyKeyError through the error path or remove the claim about the exception type.

            // PyO3 downcasts back out of anyhow, so this raises a real KeyError

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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