Create method to extract "minimal bounding" region

`spectral-cube` has a method to extract the minimal valid sub-cube, see the docs [here](https://spectral-cube.readthedocs.io/en/latest/manipulating.html#extract-the-minimal-valid-subcube). If I recall correctly one can currently extract multiple regions from a `Spectrum1D` object with `extract_spectral_region`, but that cuts out any data in between the regions. It would be good to have a method to extract the minimum bounding cube (or lower dimensional equivalent) of any provided regions.

Start by reading the linked spectral-cube minimal-valid-subcube documentation and the existing Spectrum1D.extract_spectral_region behavior. Define the API for computing the minimum bounding region across supplied regions, including lower-dimensional equivalents, and verify that intervening data is retained; the issue names no repository files or tests.

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