Make `matplotlib` a dependency?

For the spectrum utils, one of the functions uses `matplotlib`. I made it so that `matplotlib` is not a requirement. If the user doesn't have it installed, it will let them know plotting won't work. But that brings up a larger question: do we want to make `matplotlib` a requirement? I could imagine other plotting/visualization scripts that could eventually live here.

Start by reviewing the spectrum utilities function that uses matplotlib and the project's dependency configuration. Determine whether matplotlib should be required for all users or remain optional as plotting support expands. Done means the dependency policy is decided and reflected consistently in the package configuration and user-facing behavior.

Written by the indexing model from the issue text.