Update various Reference{,Set} semantics.

- Remove the inheritance semantics from `GAReference`. See #125.
- Support multiple source accessions. See #116.
- Remove `names` and `lengths` as these were only necessitated by the heavy payload on `GAReference::sequence` (as the removed comment indicates). See #120 which removes this.
- Make `GAReferenceSet::md5checksum` order-independent and remove the dependency on `GAReference::name`. See discussion on #115.
- Add record comments for `GAReference` and `GAReferenceSet`.