Spack can now be configured to assign permissions to the files installed by a package.

In the `packages.yaml` file under `permissions`, the attributes `read`, `write`, and `group` control the package permissions. These attributes can be set per-package, or for all packages under `all`. If permissions are set under `all` and for a specific package, the package-specific settings take precedence.  The `read` and `write` attributes take one of `user`, `group`, and `world`.

   packages:
    all:
      permissions:
        write: group
        group: spack
    my_app:
      permissions:
        read: group
        group: my_team