Add optional organization/group structure

### Current Behavior:
The portfolio metrics are root level. All projects are directly in the root.

### Proposed Behavior:
Add optional project organizational/group assignment where a project is a member of 0 or 1 organizations or groups. Not sure what the exact term should be.

No files or tests are named. Start by reviewing how portfolio metrics and projects are currently represented, then clarify the organization/group terminology and the zero-or-one assignment rule. Done means the agreed structure and its effect on root-level metrics are specified, implemented, and covered by tests.

Written by the indexing model from the issue text.