The Add Partition command may only add a single root node

[joswarmer]

In the #216 we decided for the bulk API that creation of a partition only creates the root node first, without children (either containments or annotations).
In the delta API the #(DeletePartition)[https://github.com/LionWeb-io/specification/blob/niko/delta-api-spec/delta/commands.adoc] takes a Delta Chunk as the new partition. Aligning this with the Bulk API, we should only allow a single node without children.

Whether we expect this, is not clearly mentioned in the delta spec, this constraint should be added..

[enikao]

In #327, we decided to allow adding subtrees for children. For symmetry, we also allowed it for partitions.

[joswarmer]

Makes sense, and for symmetry/consistency I think we should also allow subtrees in the bulk API createPartition as well.

[enikao]

Decision: We change bulk protocol to allow whole subtrees on createPartition.
Constraint: All nodes in that chunk MUST be part of the new partition, and all nodes MUST be new. We MUST NOT use implicit move semantics.

[joswarmer]

I think the text in the spec is not quite clear yet.

It shopuld day that:

• The parameter newPartition: DeltaChunk must have exactly one node with parent = null (called root node) and,
• all other nodes must be in the subtree below the root node, and
• all nodes (including the root node) must not exist already