Use pset midpoint in map projection at l2

[lacoak21]

Change Summary

closes #2582

Overview

Menlo found a bug in which some pset epoch times did not have pointing attitude coverage. This was due to a small conversion error. See #2582 for more info.

Tim/ menlo suggested using the midpoint ET of the pointing instead of the start.

File changes

imap_processing/ena_maps/ena_maps.py

• Use the midpoint time instead of the start of the pointing (epoch)

Testing

Add a small test to make sure epoch_delta returns the correct value

[Copilot]

Pull request overview

Updates ENA map pointing-set frame transformations to use the pointing-set midpoint ET (instead of start epoch) to avoid SPICE attitude coverage gaps during L2 map projection (fix for #2582).

Changes:

• Use epoch + epoch_delta/2 (midpoint TTJ2000) when deriving event_et for PSET↔Map coordinate transformations.
• Add PointingSet.epoch_delta convenience property.
• Extend ULTRA PSET test fixtures and add a basic assertion for epoch_delta.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
imap_processing/ena_maps/ena_maps.py	Switches default transform time to PSET midpoint and introduces epoch_delta property.
imap_processing/tests/ultra/mock_data.py	Mocks epoch_delta in ULTRA L1C healpix PSET test dataset.
imap_processing/tests/ena_maps/test_ena_maps.py	Adds an assertion that UltraPointingSet.epoch_delta is present and returns the mocked value.

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[subagonsouth]

LGTM