Add coordinate lookup methods to GenericSpectrogram

[s7g4]

Description

This PR adds four coordinate lookup methods to the GenericSpectrogram base class to allow for easy conversion between array indices and physical units (frequency and time).

Specifically, it implements:

• frequency_at_index(index) and time_at_index(index) for direct coordinate retrieval.
• index_at_frequency(frequency) and index_at_time(time) using a nearest-neighbor approach with np.argmin on the absolute differences.

These methods are foundational for making radiospectra more coordinate-aware and align with the goals for my GSoC 2026 project proposal to modernize the package's data structures.

I've added unit tests for all four methods in radiospectra/spectrogram/tests/test_spectrogrambase.py and included a changelog fragment.

Issue(s) addressed

• Closes Method that returns the frequency at a specific index on a Spectrogram #129

Checklist

• I have followed the Coding Standards.
• I have updated the documentation where necessary.
• I have added tests for my changes.
• I have added a changelog fragment.

Usage of Generative AI

• I have used generative AI tools as an aid in developing this code.
• I fully understand the proposed changes and can explain why they are the correct approach.

Explanation of AI usage: I used an AI assistant to help me learn and understand the codebase better. This was used as a learning and coding aid while I worked on the implementation. All of the code was written by me and checked throughly.

[samaloney]

Thanks for the PR but as discussed on the issue #129 until the underling data structure is decided this work will most likely have to be redone so it not the best use of time and effort.