Add CLI entry-point for Boresch ghost atom modifications

[lohedges]

This PR adds the ghostly command-line tool, which is an entry-point to apply Boresch ghost-atom modifications to a perturbable system, which can be used as input elsewhere.

Things to discuss:

1. The name. Feel free to suggest something better.
2. We currently only support a perturbable stream file as input. We could support the lambda = 0 state and a pert file as we do for SOMD2. If so, we'd need to apply the same trick as we do in SOMD2 to rid ourselves of the existing modifications, i.e. extracting the end states and re-merging using the same mapping.

@mark-mackey-cresset: Just tagging you in so that you are aware.

[lohedges]

The CLI entrypoints could also be renamed to something line somd2-run and somd2-ghosts if we wanted to standardise.