add support for removing dependencies

[sbailey]

This PR updates desiutil.depend to add support for removing DEPNAMnn/DEPVERnn dependencies:

• removedep(header, name): remove a single dependency by name
• remove_dependencies(header): remove all dependencies

This also required some updates to getdep and iterdep to not trip on dependencies that had been removed, resulting in non-contiguous DEPNAMnn counting.

This will be used by desihub/desispec#2121 which needs to remove DEPNAM/DEPVER keywords from individual file inputs while stacking them so that they don't get incorrectly merged by astropy.table.vstack logic which doesn't know about DEPNAMnn/DEPVERnn pairs.

[coveralls]

coverage: 74.401% (+0.09%) from 74.314% when pulling 0bd555b on remove_dependencies into 3037a6f on main.

[weaverba137]

Are you still planning to add collect_dependencies() in this PR?

[sbailey]

Are you still planning to add collect_dependencies() in this PR?

No. With the utility functions added here, the code in desihub/desispec#2101 would become

mergedep(data.meta, dependencies)
remove_dependencies(data.meta)

without needing a separate collect_dependencies function to wrap those (and two lines of "mergedep" + "remove_dependencies" seems clearer to me than wondering what "collect_dependencies" does and having to go read that code to find out...)

[weaverba137]

This all looks fine to me. Go ahead and merge when ready.

[sbailey]

Thanks. I merged and updated changes.rst in main.