Fixed recursive steiner-gauss bug

[Aerylia]

I also added some documentation and made a few refactors to make the code more legible/understandable.

The problem was that the recursive steiner tree building process required both sides of the edge in the graph to be "out of order" (aka in rec_nodes), but in reality, there will be at least 1 node in usable_nodes that is not in rec_nodes which is needed to combine the two subgraphs.

In addition to this, the vertices of the architecture are assumed to be in a post-order DFT labeling and the architecture still requires a reduce_order that has the order in which the vertices can be removed without disconnecting the graph (largest vertex first). I will write the algorithms to do this automatically later.

[Aerylia]

I also fixed that the architecture is automatically labeled according to the DFT post-ordered labeling that is assumed by the rec_steiner_gauss algorithm.
The algorithm also requires a spanning tree along which the qubits are removed one-by-one, which is stored in the reduce_order. But now, the Architecture class doesn't require that parameter any more nor a "properly labeled" graph