Support existing location IDs and custom algorithms in Lachesis postprocessing

[paco-barreras]

Why

The Lachesis changes in #403 intentionally keep the public API limited to postprocessing=None and postprocessing="dbscan". Supporting location IDs already present on the input, or arbitrary location and stop-detection algorithms, requires additional callable contracts and parameter dictionaries. Adding those options now would obscure the simple DBSCAN workflow.

Proposed behavior

Add a postprocessing option for single-user pings that already contain the configured location_id column. This path should skip location detection and call _merge_stops_from_labels with grid_based, using Lachesis's dt_max as time_thresh, min_cluster_size=1, and dur_min=0.

Also design an explicitly advanced custom option without changing the simple default API:

• A custom location detector receives a throw-away, single-user stop table plus traj_cols and a dedicated arguments dictionary. It returns exactly one label per stop, aligned by row. Missing or -1 labels must follow the same no-discard rule as DBSCAN noise.
• A custom merge-from-labels algorithm receives single-user pings containing the configured location column, traj_cols, time_thresh, and a dedicated arguments dictionary. It returns a stop table with canonical cluster labels numbered from 0 to K.
• Column-name mappings remain in traj_cols/column keyword arguments. Algorithm parameters must not be mixed into those keyword arguments.
• The public method requires postprocessing="custom" before accepting either callable. Supplying a callable with another method raises.
• Multi-user wrappers delegate the complete postprocessing operation to the single-user method and concatenate the results.
• Intermediate stop tables, detected location IDs, and mapping columns are not included in the returned table unless they were part of the user's requested output.

Acceptance criteria

• An existing mapped location_id can drive Lachesis postprocessing without running DBSCAN.
• The built-in existing-location path uses _merge_stops_from_labels with grid_based and the documented fixed arguments.
• The two custom callable contracts validate output length/type and preserve all noise observations according to NOMAD's conventions.
• Default and DBSCAN calls retain their current short signatures.
• Tests cover custom column mappings, empty inputs, one user, multiple users through the wrapper, and invalid method/callable combinations.

This pull request includes code written with the assistance of AI. The code has not yet been reviewed by a human (remove this disclosure after human review).