Add slicing to CentroidResidualsLite and add tests

[taldcroft]

Description

This makes it a bit easier to use centroid residuals from the centroids dashboard by allowing relative-time based slicing as well as normal getitem functionality. It also adds some tests.

Requires

• Further improvements to kadi observations kadi#387

Interface impacts

• Adds slicing and indexing to CentroidResidualsLite

Testing

Unit tests

• Mac

(ska3) ➜  ska_trend git:(centroid-residuals-lite-slice) git rev-parse --short HEAD
0ade2a0
(ska3) ➜  ska_trend git:(centroid-residuals-lite-slice) env PYTHONPATH=/Users/aldcroft/git/kadi pytest
===================================== test session starts =====================================
platform darwin -- Python 3.13.11, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/aldcroft/git
configfile: pytest.ini
plugins: anyio-4.12.1, timeout-2.4.0
collected 6 items                                                                             

ska_trend/centroid_dashboard/tests/test_app.py ......                                   [100%]

====================================== 6 passed in 3.37s ======================================

Independent check of unit tests by [REVIEWER NAME]

• [PLATFORM]:

Functional tests

No functional testing.

[jeanconn]

This comment looks stale to me and it won't let me do an in-line comment ...

Munge the yag/zag data type and trim by duration if requested.

[taldcroft]

Fixed the "munge" comment.

[jeanconn]

For this application I think this slicing code is OK, but if we start using this other places it may need more review.