LSRK<->TOPO use case example

At least one use case that is reasonably common is this: spectra are taken in the topo frame over a range of times, often different nights or months. They are shifted to a common frame (LSRK or barycentric, doesn't matter, but LSRK is the standard choice in radio), stacked, then analyzed to select frequency ranges that contain lines. Those frequency ranges then need to be shifted back to the topocentric frame (at each observation time) so the line-containing regions can be masked out to measure a pure continuum.

The issue describes a workflow for shifting spectra between topocentric and LSRK or barycentric frames, but it names no files, tests, or implementation entry point. First clarify whether the request is for a new transformation, an example, or documentation, then define the expected behavior and tests for selecting and masking frequency ranges across observation times.

Written by the indexing model from the issue text.