Spinoff environments (inheriting from the base environment) for every use case / user group

[zacharyburnett]

In order to prevent the base stenv environment from becoming the kitchen sink for all packages (and inevitably having to manage inter-dependencies and conflicting requirements), it might be useful to have a minimal base environment with packages common to all users, and then a few spinoff environments that inherit the base environment but add more packages that are useful to a certain group of users / certain use cases.

These spinoff environments might be focused on individual missions, observational modes, etc, and would be free to include any version pins / backward requirements on the base packages.

I imagine the implementation of this would be

1. a set of YAML files in a directory called spinoff or something like that
2. a corresponding addition to the GitHub Actions CI workflows that build, test, and export the spinoff environments along with the base one